Let the site a short form URL gets expanded into be customised

[MageJohn]

Overview

Right now if you want to use something other than GitHub over HTTPS to get plugins you need to specify the full URL; if you want to do this with more than one or two plugins it will get messy. This PR adds a two keys to the config dictionary for minpac#init and one key to the config for minpac#add that try to fix this problem.

Use cases

• Change all your plugins to update over SSH.
• Remove all long form urls and just pass an extra key to minpac#add, which makes for a cleaner vimrc/init.vim
• Future proofs the plugin; if the plugin ecosystem changes to no longer have most plugins on GitHub, no update to minpac is required to support it.

What's been added

The keys for minpac#init are 'site' and 'sites'. 'sites' is a dictionary whose keys are short names for sites, and whose values are the base url for the corresponding site; the default names are include 'github', whose value is 'https://github.com/'. See the code for the others. 'site' is a key from the 'sites' dictionary, default 'github'. This is used as the default site for short form URLs passed to minpac#add.

The key for minpac#add is also a key from the 'sites' dictionary, and overrides the value set by minpac#init.

I've also added tests for the new configuration options.

To-do

This will probably need documentation before it's merged.