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.
Note that this does NOT port the following command over:
```shell
git config --global protocol.file.allow always
```
Doing so would change the git configuration of users running
the unit tests locally, and this is not an acceptable outcome.
Instead, the git configuration is modified at the CLI using
the `-c protocol.file.allow=always` argument to accomplish
the same thing without side effects.
