Document --file-filter

[Oxymoron290]

please document --file-filter

https://github.com/acaudwell/Gource/wiki/Controls

I'll probably try to do a PR soon but I am running short on time lately.

[bitpaint]

--file-filter REGEX: This option is used to exclude files whose paths match the provided regular expression. If a file path matches the regex, it will not be included in the visualization.

--file-show-filter REGEX: This option, on the other hand, is used to only show files whose paths match the given regular expression. Any files that don't match the regex will be excluded from the visualization.