Feature flag

[mtzguido]

F* routinely gets new features and there's no good programmatic way of testing for them. We could support something like:

$ fstar.exe --list-features
fstar.include
context-pruning
...

and

$ fstar.exe --has-feature fstar.include
yes (or exit with zero/nonzero)

And add more as new features are included. This would allow writing better makefiles/workflows since we can check for specific named features instead of version numbers or similar.