Allow blank filename lists so that ignore files are respected

[styrmis]

Specifying '*' will cause any local ignore files to be skipped.

When all (non-ignored) files should be searched then no files should be specified for the grep program.

When not using a grep program, an error will still be raised if we end up with an empty list of files.

Tested to work with ag and rg.

References #6

[styrmis]

@yegappan Would it be possible to merge this? Happy to alter the implementation if you have feedback on it.

Still using this plugin regularly, now with rg, thank you for publishing it 🙏

[yegappan]

Thanks for the patch. Can you update the help text with a description about this?