Glaive feature to trigger plugin activation

It's a common pattern to install a plugin, configure it, install another plugin, configure it, etc. It would be nice to be able to consolidate this down to a single line by configuring `:Glaive` to hook into your plugin manager somehow so it knows how to trigger plugin activation.

The [vim-pi](https://github.com/MarcWeber/vim-addon-manager-known-repositories) project (vim plugin index, a spinoff of the vim-addon-manager project) maps canonical plugin names to an installation source. This means we can both install and configure the plugin using the same short canonical name. For VAM, it's as simple as telling `:Glaive NAME` to execute `:ActivateAddons NAME` if the plugin isn't yet activated. For other plugin managers, it might be more complicated, but should still be possible.