Improvements

[michaelhogg]

Many thanks @karlspalding for creating this plugin!

I've made a few improvements:

• 4eea3cd – No parameters should be passed when calling Promise.resolve(). See here, here and here.
• 7f943d9 – Instead of calling sassLint.outputResults() to output the error results directly to the console, sassLint.format() should be called to generate a string containing the formatted error results which should then be passed to Promise.reject() for Brunch to output to the console. See here, here and here.
• fa35cac – Fix bug involving plugin config default values. If config.plugins.sassLint is provided, then the default values for both file and options will be lost. For example, if config.plugins.sassLint specifies options but doesn't specify file, then the plugin won't supply a default value for file. An implementation which avoids this problem can be seen here, here and here.
• 2936f33 – Implement new config option: warnOnly. This is a useful feature which has been implemented in other Brunch linting plugins (see here and here).

[karlspalding]

Thanks for the PR, @michaelhogg.