Proposal to add skpm plugins
Hi Mathieu,

I want to add a template to skpm based an improved version of [NibUI](https://github.com/aparajita/Sketch-NibUITemplatePlugin). An essential part of making the template work is a task that exports the xib to a nib and also writes out connection info into a file.

Since this task should logically be part of the skpm watch process, I propose to add a simple plugin interface to skpm/skpm-build that would allow templates to provide hooks into the various lifecycle points, e.g.:

- create
- build
- link
- watch
- publish

What do you think? It's something I'm willing to work on.