Cargo errors after deleting overridden directory

If an override is created for a directory, and that directory then removed, any subsequent calls to cargo or rustc will cause multirust to display an error message before executing. This can only (currently) be resolved by recreating the directory, removing the override, and then removing the directory again.

```
alex@endurance:~> mkdir testing
alex@endurance:~> cd testing/ ; multirust override stable
multirust: using existing install for 'stable'
multirust: override toolchain for '/Users/alex/testing' set to 'stable'
alex@endurance:~/testing> cd .. ; rmdir testing/
alex@endurance:~> rustc
/usr/local/bin/multirust: line 1039: cd: /Users/alex/testing: No such file or directory
/usr/local/bin/multirust: line 1039: cd: /Users/alex/testing: No such file or directory
error: no input filename given
```

Proposed solution in PR #91, allow for users to optionally name the override they would like to remove, so the recreation of the directory is no longer necessary.