Support gulpfile description through use of .gulprc file

[phated]

Must wait for #69 to be implemented.

Moved from gulpjs/gulp#1534

I'm envisioning this as a description property in the .gulprc file that replaces https://github.com/gulpjs/gulp-cli/blob/master/lib/versioned/%5E4.0.0/index.js#L57, https://github.com/gulpjs/gulp-cli/blob/master/lib/versioned/%5E4.0.0/index.js#L49 and https://github.com/gulpjs/gulp-cli/blob/master/lib/versioned/%5E4.0.0/index.js#L67 (and corresponding versioned files).