Add DiffSplitHunk function.

Splits the hunk in which the cursor is making sure that all surrounding
hunk headers are correct (i.e., adjusts the original hunk header).
Add DiffDeleteHunk command for deleting current hunk.

Deletes the hunk on which the cursor is and makes sure that the
following header is correctly changed.
Add documentation.
Don't redefine major vim commands.

Instead of redefining D and S, use <leader>-prepended mappings instead.
Also fix header of the file.