Make Speasy more modular with plugins

[jeandet]

The entry point specification looks like a nice way to allow third party Speasy plugins.
We can also think about moving some providers away into plugins in the future.

[jeandet]

Note that doing so, implies that we won't test anymore change in Speasy core together with external providers.
That means that we should have a well specified and tested Speasy API and we have to be extra careful with declaring compatible Speasy version in plugins.