Move coffeelint settings to external file

Depends on atom/grunt-coffeelint#2

The motivation is for this is to allow `coffeelint` config to be shared by
`script/grunt coffeelint`, global `coffeelint` and inline linting via
`AtomLinter`.

Test Plan:

Made some deliberate lint errors then ran `script/grunt coffeelint` and
verified the output looked the same with and without this change.
sure, done. probably want to remove that once #3116 lands though