Feature/agnostic commands

The current setup does not allow users to specify their own mappings without modifying the plugin itself, as <Plug> in their vimrc context will not reference the vCoolor plugin. Instead, I added support for direct commands `:VCoolor` and `:VCoolorR`, and updated the documentation accordingly.

This is important for adding OSX support, as it will require somewhat different bindings.

As a general rule, I would recommend removing the default bindings and providing them in the README, as most Vimmers tend to want to use commands in their own ways, but that's more of a philosophical viewpoint.