feat: toggle off repo sync completely

[jthegedus]

Summary

This builds on #957 by allowing the plugin shortname repo sync to be toggled off completely with:

# $HOME/.asdfrc
plugin_repository_last_check_duration = never

Fixes: #978

Other Information

Docs changes comparison

[jthegedus]

I am still not happy with the docs for these. I will propose another change tomorrow.

I think the tables make each of the valid values and their affect more clear, especially in the case of plugin_repository_last_check_duration where we now have 3 different settings.

[Stratus3D]

Sorry for the late review. Looking at this now.

[Stratus3D]

Tables seem a little unnecessary for some of these options.

Also, this:

Delete source code or binary after install

Should be:

Delete source code or binary after successful install

[jthegedus]

Tables seem a little unnecessary for some of these options.

I found the the plugin_repository_last_check_duration very wordy and unclear without the table. After adding one it seemed best to be consistent, though I agree the binary settings don't benefit all that much.