use a setting to customize fileWatch interval for autocompiling

[jvanasco]

right now the app checks files using a 500ms fileWatch interval for detecting changes in autocompiling

it would be great if this were configurable in the settings.

rationale: on large projects with many includes, this can generate a bit of a performance hit. Dropping this to something in the 10s to 2s range can make a noticeable difference.

The relevant files are-

https://github.com/oklai/koala/blob/master/src/app/scripts/fileWatcher.js#L201
https://github.com/oklai/koala/blob/master/src/app/scripts/fileWatcher.js#L239
https://github.com/oklai/koala/blob/master/src/app/scripts/projectSettings.js#L305