Add --include option

Repositories that include many non-source files can be more easily analyzed by passing arguments to `git ls-files`. This allows a user to whitelist specific filetypes or directories (eg. --include src/*.py), instead of individually blacklisting all non-relevant files with --exclude (eg. --exclude "res/,lib/,third-party-lib-1/,third-party-lib/2,...").

Note: git-fame will process the intersection of the sets of files allowed by --include and --exclude.