add command line option to write to file

[dmashine]

def print_messages(self, write_to=None):
   write_to = write_to or sys.stdout

write_to is already defined. Please add a command line option (like prospector --write-to=file.xml) so output will be in this file.

it is needed, because tox dont support pipeline redirects and someone may want to output file in xunit format.

[tysonclugg]

I'd like for prospector to be able to write xunit output to a file, in addition to showing grouped output on stdout. Perhaps with invocation like this:

prospector -o xunit:reports/prospector.xml -o grouped

[chocoelho]

@tysonclugg you mean just like pytest-cov does? That'd be great.

[carlio]

That PR is merged now so should be working!