Add `mocha` option to allow overriding Mocha

This feature allows users to supply the plugin with a version of Mocha
of their choosing. This means the latest version of Mocha can be
injected without needing to update this task, and older versions of
Mocha can be used in test environments that depend on deprecated
behavior.
Merge branch 'specify-mocha' of https://github.com/jugglinmike/grunt-mocha-test into jugglinmike-specify-mocha