Add plugin system

Add possibility of defining new plugins and configuring and running them at various stages of Télépherique. This might require an interface with callbacks or something alike.

Example: plugin for setting up LMDZ output fields. Normally, this requires a lot of changes in the XML XIOS configuration of LMDZ. But, in reality, this could as simple as just:

[[plugin.lmdz_output]]
field = "t2m"
frequency = "1H"