Add filename information to format command line option

[mnojek]

There is a possibility to include file path in issues reported in output. Special FORMAT option can handle the appearance of the results and it already has a possibility to display path to the file but it's not documented in the help info. This Pull Request adds this info to the help message.

I find it very useful, because that way I can imitate the output to be similar with the pylint, thus I can take advantage of the Output filters feature in External tools in PyCharm as well as parse the output easier when integrating RFLint with CI&CD tools.