Update lsc.js

add joinWithBare option to 'joinThenCompile'
---
Hi,
I'd like to purpose an option to be added. I named it 'joinWithBare'.
What it does is before join each .ls files, let them wrapped into a 'let' block.
Thus can prevent variable pollution for each seperated '.ls' files.
I wrote it on Github so maybe u'll be interested in rewrote them with tests. Thanks!!