Improve path exclusion

[marclop]

Description

Currently path exclusion is very raw and one could argue that incomplete. There's a few issues to solve here:

• Path exclusion currently supports directories rather than filenames. It'd be useful to be able to also skip certain filenames if specified in the -exclude flag.
• Potentially support a regex or

Code detail

https://github.com/elastic/go-licenser/blob/master/main.go#L199-L207