helper script: attach
Currently, attaching a file to a task is accomplished like so;

```
task 10 annotate ref: ~/documents/things/this_is_the_thing_to_be_attached.txt <enter>
```

but there exist several ways to invoke a file-browser to do the same thing. 
This feature suggestion (for an external, but related script) seeks to allow you to use your system-default file-open dialog, or a user-configurable alternative, instead of remembering, or cut-and-pasting that path/file.

(I like ranger for this; http://ranger.nongnu.org/, see --choosefile=targetfile option)
so the above command would be accomplished with

```
attach 10 ref: <enter> (browse to file) <enter>
```
