Allow multiple output 'type's for code coverage

The coverage should be reworked to treat `type` as an array of report output formats...

So that at this point in the code, multiple output formats can be written.
https://github.com/vojtajina/testacular/blob/master/lib/reporters/Coverage.js#L96


I haven't looked too closely at how reports are generated, but as I understand it, all the collection has completed, and reports are simply the way to format the output.  I also verified that Istanbul doesn't allow arrays for `type`, just a string:
https://github.com/gotwarlost/istanbul/blob/master/lib/util/factory.js#L41


Should be a straightforward change to check the param type and either generate one or many reports I think.

/cc @taichi