Add better grep() support to js api

[tj]

string or regexp

[davemckenna01]

I'd like to help out with this. What did you have in mind: something like

mocha.setup({
  ui:'bdd',
  grep:'foobar'
});
mocha.run()

or more like

mocha.grep('foobar').run()

[tj]

I prefer the latter personally

[danielstjules]

Looks like this was completed in #407 as well!