allow splitting and merging of heading cells

I consider it a bug that you couldn't merge with heading cells, and that you
couldn't split them, either. (So much so that I thought it was a bug in
ipython-vimception when I ran into it).

This change removes that limitation, so heading cells are on par with the
other cells in terms of the kinds of manipulations one can carry out with
them.

Pinging @minrk, since `git blame` told me he had added the code to disallow
merging and splitting with heading cells.