Accept string representing regex to filter generated files without modification

[ctumolosus]

It's unclear to me which syntax the CLI wants me to use to filter files. It looks like it wants to support globs, but the implementation to turn a glob into a regular expression feels incomplete. At least, I could not find a way to exclude a single file by name. Since the patterns that can be used are poorly documented and feel limited by the implementation, I propose the file filter should accept a regular expression without substitution. In my case, this would allow me to exclude a single file using a negative lookahead ^(?!filename).*\.js$.

Basically, I'm proposing this line is changed from:

file_regex = Regexp::new("^" + filename.gsub('.', '\.').gsub('*', '[^.]*').gsub('?', '[^.]?') + "$")

to:

file_regex = Regexp::new(filename)