Dynlink with Findlib support for long-running plugins

In the config file

```toml
[plugins]
   ocamlfind = ["some-package", "some-other-package"]
   compiled = "some_dyn_compiled_module.cmxs"
```

Ketrew will load at runtime the findlib packages and the compiled module.

There is one example of plugin: `src/test/dummy_plugin.ml`  and a workflow that uses it `dummy_plugin_user.ml`.

Documentation will come in a later PR (other things to document).

Issue #1 is known already; saved for later.





Yes, in the branch `moadoc` (the next PR) there is more documentation about the configuration-file:

https://github.com/hammerlab/ketrew/blob/moadoc/src/doc/The_Configuration_File.md

And I added this: https://github.com/hammerlab/ketrew/issues/6