Add option to skip rebuilding and load prebuilt plugin configuration

[andreyorst]

We should detect if the plugin has an entry in the build directory and skip the build process altogether. The rebuild can be invoked manually with plug-rebuild [<plugin>] command. If no plugin is given, rebuild all plugins.

This has a potential problem of the build being unsynchronized with what's in the user's kakrc, so this should not be on by default and controlled with plug_auto_rebuild option.

Rebuilding can be probably triggered via a hook on saving the buffer to prevent configuration desync. Such hook should be set up by the user.

[mralusw]

You could also save the list of .kak files to be sourced for each plugin, to avoid the numerous find's. I believe this is what cork does.. A git post-update hook could create a .build/repo/outdated file (which, when present, triggers a rebuild) — or something like that.

As far as the kakrc sync problem you mention, it's more complicated, since kakrc can source other files which in turn call plug (e.g. I have an autoload.parts directory which sources files in Linux run-parts order).