Feature write to file

[chocoelho]

Resolves #246.

As of now, there's no way to way to redirect output to files. This implementation was inspired on how it was described on #246 and how pytest-cov redirects its output. It's also possible to set multiple outputs for the same export, so if one runs prospector -o xunit:output_file.xml:/home/xpto/output_file.xml it'll write to both relative output_file.xml and absolute /home/xpto/output_file.xml. Multiple reports are also supported, so prospector -o xunit:output_file.xml -o grouped will write XUnit format to output to relative output_file.xml and grouped format on console.

[dmashine]

@chocoelho please can you merge it to base branch? I like this feature very much

[chocoelho]

I'll try to resolve this and a couple of other PRs in the next couple of days. They're just missing some test cases and, probably, a few other adjustments.