Fix Some Review Comments

[ripytide]

Fixing some of the comments raised in #23

PR is not quite yet finished but I have finished enough work for today and wanted to put my PR up as a draft in case anyone wants to look/comment/work/interact on it.

• groups now defaults to empty if you have no groups folder
• added a new hostname_groups_enabled config
• empty backends are now not outputted from pacdef unmanaged
• implemented a new pacdef add command with already-in-your-group-files error messages and using toml_edit with a default default.toml group to save having to always pass three required arguments (now just two required and one optional)
• added a new --include-implicit option to the clean and unmanaged commands and made it only show explicit packages by default

[ripytide]

@InnocentZero I am merging this without review since I'd like to ask if anyone from the upstream pacdef project wants to try out this version.

[ripytide]

Feel free to review it anyway though if you want and we can resolve comments in another PR