Switch to cson-safe for parsing cson

This is a lot safer (no more execution of random CoffeeScript).
Plus, it gives better syntax errors. 

See https://github.com/groupon/cson-safe.

The atom guys https://github.com/atom/season already switched, so we should as well.

Note that this may **break** some grunt configuration with lazy/bad `.cson` syntax (see `config/markdown.cson` of this commit), so this should be a major update.