Added backup option (defaults to false) which will cause dotbot
to backup already existing files by renaming them with suffix
.dotbot-backup. Added unit tests and updated README.
See https://github.com/anishathalye/dotbot/issues/282 and
https://github.com/anishathalye/dotbot/issues/315.
This patch simplifies the implementation, removing special-case handling
for the cases of zero matches and one match. Instead, any situation
where `glob: true` is specified and the path contains a glob character
(any of "?", "*", or "[") is treated as a glob case. The reason we check
both `use_glob` and `_has_glob_chars()` is to more gracefully handle the
case where the user has enabled globs by default, but most links do not
contain glob characters and should not be treated as globs.
