Implement indent-region for haskell-indentation-mode

[gracjan]

Indent region should align indentation according to current settings without changing program semantics.

[gracjan]

This would not help as usually the scenario is:

1. Alter first line somehow.
2. Call indent-region to fix remaining lines.

Note that between 1 and 2 program is already semantically broken and knowing what is going on would require time travel.

Add this info to documentation somewhere.