Add --path to redub use

[LunaTheFoxgirl]

I install my own compilers into my own paths, and would like the ability for redub to either:

1. Accept a name if it can be found in PATH if no version is specified
2. Allow setting the version part of the command to a path to the compiler to use, or have an option --path option to specify a path.

[MrcSnm]

Updates:

• Redub now supports redub --compiler=ldc2@1.42.0 or even semvers such as redub --compiler=ldc2@>=1.42.0. Though it does not uses a best match algorithm for that.
• You can also now use redub use ldc2 some/path/to/a/ldc/binary
• redub use ldc2 will now look into your system PATH by default first. If not found, it downloads latest by default.