Add keywords to the REPL autocompletion list.
Add keywords to the REPL autocompletion list.

In particular, the following keywords will now be suggested by
the REPL autocompletion:
  else, if, let, then, where

These, I believe, are the only keywords which can be entered at
the command line.

Fixes #144

GaloisInc/cryptol,GaloisInc/cryptol,GaloisInc/cryptol