[feat request] option to ignore/include files using rsync-filter syntax

[renatocoutinho]

The idea is to add the ability to pass "-F .rsync-filter" and have it ignore the files inside (using rsync's syntax). The rsync syntax could probably be rewritten as a big (set of) regex.

Reasoning: I often use dust prior to backing up messy directories with rsync, and exclude some files/dirs using rsync filters, then I re-run dust excluding all those files/dirs by hand - if dust had this feature, the process would be a lot easier.

rsync does something more complicated with only "-F" which is to recursively check for .rsync-filter in every sub-folder, that does look more complicated to implement.