Issues when dealing with large Sketch files
## Description
I've been testing Kactus out as a potential option for our Design team, and tested with two different `.sketch` files (one is 275MB and the other is 35MB).

When dealing with the larger file, I encountered a similar issue to https://github.com/kactus-io/kactus/issues/89, and eventually that repo became unusable in Kactus.

When dealing with the 35MB file, even a small change that results in only one JSON file being updated, takes almost 2 minutes of processing within Kactus to detect the changes, and uses 2.20GB of memory. _Note: the 2 minutes processing time is _only_ to realise there is a changed file, and does not include the time to generate the previews._

Unfortunately I'm not able to share the Sketch files publicly, but I can give some details below on the 35MB file:
- there are 7 pages, with the following directory sizes (in the JSON files generated by Kactus) 2.3MB, 1.1MB, 1MB, 8.3MB, 336KB, 13KB and 694KB
- when unzipping the `.sketch` file and looking at the JSON (different to the JSON generated by Kactus), there is one JSON file that is 280MB (not a typo).

## Version
* Kactus: 0.3.6
* Operating system: macOS 10.13.3 (Processor: 2.9 GHz Intel Core i7, Memory: 16 GB 2133 MHz LPDDR3)

## Steps to Reproduce
1. Create a new repo with a large Sketch file (around 35MB)
2. Commit the initial Sketch file
3. Move a single group of elements to different coordinates within the same Artboard
4. Open Kactus

### Expected Behavior
Kactus detects the changes in a reasonable amount of time.

### Actual Behavior
Kactus takes a very long time to detect the changes.

### Logs
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The logs related to the issue I've described with the 35MB file are the following:
```
2018-06-04T12:33:44.499Z - info: [ui] Executing exportCommit: git archive 1710261f8b4da3471b8410cd6b99f5953b643f00 -o /Users/dylan.lewis/Library/Application Support/Kactus/previews/6/1710261f8b4da3471b8410cd6b99f5953b643f00.tar (took 2.498s)
2018-06-04T12:33:56.315Z - info: [ui] [AppStore] loading 6 repositories from store
2018-06-04T12:33:56.316Z - info: [ui] [AppStore] found account: asos-dylanslewis (Dylan Lewis)
2018-06-04T12:33:57.474Z - info: [ui] launching: 0.3.6 (Mac OS 10.13.3)
2018-06-04T12:33:57.474Z - info: [ui] execPath: '/Applications/Kactus.app/Contents/Frameworks/Kactus Helper.app/Contents/MacOS/Kactus Helper'
2018-06-04T12:34:10.182Z - info: [ui] No submodules found. Skipping "git submodule status"
2018-06-04T12:34:18.128Z - info: [ui] Executing [Kactus] generate page preview (took 19.521s)
2018-06-04T12:35:12.610Z - info: [ui] Executing [Kactus] parse file 180523_Master_sketch_file_v1 (took 4.941s)
2018-06-04T12:36:37.171Z - info: [ui] Executing getStatus: git status --untracked-files=normal --branch --porcelain=2 -z (took 2.326s)
2018-06-04T12:36:43.925Z - info: [ui] Executing exportCommit: git archive 1710261f8b4da3471b8410cd6b99f5953b643f00 -o /Users/dylan.lewis/Library/Application Support/Kactus/previews/6/1710261f8b4da3471b8410cd6b99f5953b643f00.tar (took 1.034s)
2018-06-04T12:37:00.635Z - info: [ui] Executing [Kactus] generate layer preview (took 23.419s)
2018-06-04T12:37:06.193Z - info: [ui] Executing [Kactus] generate page preview (took 23.937s)
2018-06-04T12:37:06.284Z - info: [ui] Executing [Kactus] generate layer preview (took 23.402s)
2018-06-04T12:37:06.298Z - info: [ui] Executing [Kactus] generate layer preview (took 22.640s)
2018-06-04T12:37:06.751Z - info: [ui] Executing [Kactus] generate layer preview (took 21.912s)
2018-06-04T12:38:58.594Z - info: [ui] Executing exportCommit: git archive 1710261f8b4da3471b8410cd6b99f5953b643f00 -o /Users/dylan.lewis/Library/Application Support/Kactus/previews/6/1710261f8b4da3471b8410cd6b99f5953b643f00.tar (took 3.781s)
2018-06-04T12:39:34.278Z - info: [ui] Executing exportCommit: git archive 1710261f8b4da3471b8410cd6b99f5953b643f00 -o /Users/dylan.lewis/Library/Application Support/Kactus/previews/6/1710261f8b4da3471b8410cd6b99f5953b643f00.tar (took 2.560s)
```