Add :Driver command to re-source user specific rc

[pcapel]

What

A convenience function that allows you to quickly source a configuration file scoped to your own handle (e.g. .vimrc.pcapel).

Why

Sometimes working using another developer's configuration options can be frustrating, particularly when they affect the native defaults. This is a pain point for both developers, and this could act as a convenient compromise.

[pcapel]

So how would you reset? Just re-source vimrc again?

Probably something like adding a default argument makes sense. The default could reasonably be the .local file. The idea was to allow people who have more particular preferences to pair easily. A command run when you take over driving seems fairly unobtrusive, but having one to reset to the standards would also be good.