Improve interface for ignoring files

[reitzig]

Option -i can be used to ignore individual files. It is arduous to use for long and/or changing lists of files.

Add an option that takes one or multiple ignore files; each line would correspond to a file name (pattern) to be ignored.

Goal: support .gitignore and similar files. Check for libraries that parse such, in particular considering they may contain glob patterns. Candidates:

• ruby-doc.org/core/File.html#method-c-fnmatch
• https://github.com/adamsanderson/file_glob

Refactor out own ignore file (format) while we're at it.

TBD:

• Make use of .gitignore by default? Are there other canonical candidates?
• Introduce a canonical file name for a user-specified ignore file, similar to .gitignore?

See #89 for a use case.