Provide a simple to use plugin system

[mstruebing]

ASDF uses bash scripts which need to be written all on their own.
We should at least provide some commonly used functions in a library which can be used in that plugin directly i.e. download the GitHub release.

I'm not sure if we want to stick to bash, in my opinion, writing bash scripts can be hard.

We could even define a standard way of loading plugins and let the user provide for example a single repository name if the tool sticks to our standards i.e. version scheme, architecture names in the release etc.

[aweris]

For the plugin system we should add one more layer for the source and re-use asdf as a source to use existing plugins and migrate easily from our repositories