Create preview size config plugin

Prompt the user for their desired preview size, and write their
selection out to a plist in the cache directory. Less transient
locations seem to not play well with sandboxing, so given the ease of
re-selecting the desired preview size, this is good enough.

It might also be desireable to query Sketch for the possible preview
sizes, but we'll do that if/when the possible values ever change.
Use config for preview size
Remove semicolons
Move common code to common.js

I don't really like duplicating code like this, but I think it's worth
it for now to keep the double-click to install ability for users. It'd
be nice if plugins could be distributed as a bundle, but this doesn't
seem to currently be possible.
Remove Sketch 2 support

MSExportRequest, which is used to resize preview images, does not exist
in Sketch 2, so it's time to remove support.
Update README
Update CHANGELOG for release