ignore-dot-files: add ignore_dot_files option

I had an issue where whenever I save in Emacs, it briefly created a file `.#<filename>` for a given `<filename>`. This ends up matching the regex, but by the time guard attempts to open the file, it's been deleted, which ends up getting the `guard-asciidoc` watcher fired (removed) from the guard process. This creates an option to ignore dot files (fairly specific, but a reasonable request in my opinion.)

There is likely a better way to do this, and it may actually be more general to allow for a custom regex, but I'm don't dabble with Ruby very often, so this is the best I came up with. If you'd like to suggest a different way, I'd be happy to try and implement it and amend this PR.