move mergeopt to utils

from Cell
make default cell type configurable

- adds IPython.notebook.default_cell_type
- default is 'code' (matching IPython 2.0)
- special heuristic values include:
  - 'selected': default to selected cell (current master behavior)
  - 'above': default to cell above
  - 'below': default to cell below
Merge pull request #6408 from minrk/new-cell-code

make default cell type configurable