feat: add opts option and default behavior

[bemble]

In reply to the issue #44, I add the ability to get the default options from a mocha opts file.
By default it takes the default path of mocha, test/mocha.opts but users can define another path thanks to the opts option.
Last thing, it still working if the file is not found.

[sindresorhus]

As commented in #44, this should be fixed in Mocha, not here.

[bemble]

@sindresorhus I totally agree with, but for now I see two points: as you probably already know, the way mocha handles opts is quiet dirty (shortly, the opts file is read and the opts are added to the current process so mocha standard behavior can work) so we have to wait until they refactor all that part, which brings me the second point, we don't know if they will refactor this, soon or not, according to the issue here mochajs/mocha#1457.

[sindresorhus]

Alright, but least make it a module that can be depended upon here so I don't have to maintain all this code.

[shellscape]

closing on age and last comment by @sindresorhus