Prepare 0.5.0 release
Merge pull request #6 from mndrix/prolog

Improvements for Prolog support in Atom
Prepare 0.6.0 release
Increase indent for DCG and if-then-else

Standard Prolog indentation rules dictate that goals inside a DCG and
the then clause inside an ->; should be indented.
Handle must_be/2 type declarations

A must_be/2 goal asserts that the second argument must have the type
described by the first argument.  Its behavior is semantically similar
to type declarations in other languages, so it should use the same name.