Add failOnError option to play nice with watch

Make development time less frustrating.  I like to do a full build before I setup [watchers](https://github.com/gruntjs/grunt-contrib-watch) in development.  

Currently, If I have a stray syntax error in a stylus file when I fire up my development environment grunt dies on the full build before the watchers are setup.  I go fix the error, then have to manually start the development task again. 

With this change, grunt still logs the stylus syntax error to help me troubleshoot, but happily carries on to setup watchers and rebuild styles as soon as I fix the error.

Inspired by [grunt-shell](https://github.com/sindresorhus/grunt-shell#failonerror) [and](https://github.com/stephenmathieson/grunt-jslint#documentation) [others](https://github.com/webpack/grunt-webpack#configuration-example).

Fixes #51.  Alternative to #52.