Filter zero-length strings from mocha.opts

Fixes mochajs/mocha#1471
Clean up unused module dependencies.
Merge pull request #1474 from monowerker/topic/mocha.opts-exception#1471

Topic/mocha.opts exception#1471
showDiff: don’t stringify strings
Merge pull request #1477 from marcello3d/showdiff-stringify

showDiff: don’t stringify strings + tests