Add backend plugin support

[AltGr]

Implements #209.

[AltGr]

What should I do with this ? I propose the following:

• remove patch 9da75f7 which contains just demo plugins (esp. the python one is of no interest since the backend is already built-in !) ; another PR to re-add the jsoo plugin after a little cleanup should be fine
• rebase the rest and merge ? In particular a212d52 has some refactorings that are already conflicting and will get worse as time goes
• if more debate is needed on plugins, maybe rebase a212d52 as a separate PR. I'll in any case have to extend the debug_printf functions for consistency across the various printing functions ; nothing difficult but it should be done and merged quickly to avoid losing time on conflict resolution :)

[AltGr]

Rebased ; the first patch should be fine, the second adding the demo plugins needs some cleanup (removing the dummy python plugin, decide where/how we put the jsoo one, add it to the normal build path…)

[AltGr]

Rebased, I think it should be OK now, and the two plugins are properly advertised as examples.

[denismerigoux]

It looks good to me code-wise but I think this plugin system is lacking some high-level documentation.

1. Can you define a plugin.mld file in compiler/plugins ? Would this file be showed in the odoc output ?
2. Can you write in CONTRIBUTING.md a guide on how to define a new backend plugin and make it work ?

[AltGr]

I'll do that, thanks!