Reading depopts from OPAM file

Plus some revised documentation for DEPOPTS and HOOKS.
would be better if we can select a subset of DEPOPTS manually I think. ie. by default, we use your runes but if DEPOTS are specified in the script then they will be used instead.

I really want to have something like this working correctly:

```
env:
   - DEPOPTS="lwt"
   - DEPOPTS="async"
   - DEPOPTS="lwt async"
```

What do you think?