Define mmm-indent-line-function and its default value

* Now html-erb-mode can just set mmm-indent-line-function.
* Easier access to the submode's indent-line-function. No need to hack around.
* mmm-indent-line indents according to the submode at the indentation, which is more
  consistent than previously.
Merge pull request #22 from purcell/indentation

Merge branch 'indentation'