To try it (without installing Catala):
```shell-session
$ make plugins
$ export CATALA_PLUGINS=_build/default/compiler/plugins
$ dune exec -- catala lazy examples/aides_logement/tests/tests_calcul_apl_locatif.catala_fr -s Exemple2
```
Keep in mind that this is a work-in-progress prototype :)
Fixes#378
- the plugins are compiled as libraries rather than with `executable`, so that
dune is able to install them
- they get installed to `lib/catala/plugins/<plugin-name>/<plugin-name>.cmxs`
- the lookup for plugins is now recursive to cope with the plugin subdirectories
in the point above
- one that is just the same as the python backend
- one that is a quick'n'dirty call to js_of_ocaml on top of the ocaml backend to
generate js files
Use `make plugins` to test them. They are only expected to be useful as
demonstrations of how to write plugins.
