Improve path exclusion

## 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

Start by reading main.go lines 199-207 and tracing how the -exclude flag is parsed and applied. Review the existing path-exclusion behavior and determine the expected filename support; the regex idea is only proposed, so its scope and acceptance criteria need clarification before implementation.

Written by the indexing model from the issue text.