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

See the commit.

Drawbacks:
1) `mmm-update-submode-region` will make default indentation a bit slower.
2) If some people preferred to indent the closing line of a multiline subregion by first navigating to the eol (where it's back to the primary mode), it won't work now. That approach is definitely flawed, though.

Thoughts?