Support mid-line Enter

[tmcw]

In this situation

/**
 * Comment
 * Something <cursor> something else
 */

It'd be nice for Enter to produce:

/**
 * Comment
 * Something
 * something else
 */

This should handle all of the properties of our existing continue system - it'll preserve surrounding indentation, so if this comment inside of a function so it's indented 2 or 4 spaces, it should preserve that. Plus, we may want to truncate the whitespace at the end of the line? That's optional.

[mattmundell]

Done in /pull/9.