Setup for use *as* a GitHub action

I would like to suggest setting this repository up so that it can be used *as* a GitHub action. This would involve a yaml config file for the action and a Dockerfile that sets up a copy of VIM + NeoVIM (configurable?) and this plugin and makes it easy to run against a vim plugin repository (i.e. setup the `rtp` etc.)

I see a lot of projects struggling with how do manage this in CI jobs. It can be done (for example see one of [my implementations here](https://github.com/ledger/vim-ledger/blob/master/.github/workflows/vader.yml)) but all that boiler plate could basically be done on this side leaving the project with a one liner to run their test suite.

I realize maintenance is pretty low right now, but per #221 I am suggesting migrating this plugin to the @preservim org. This is one thing I would be able to contribute it if were migrated. Alternatively if somebody is going to commit to reviewing and merging PRs I would be happy to submit it here too. Finally it could be setup as a third party repository that basically wraps this one, but I do not think that is ideal.