Face to highlight active cell + expanded defaults for cell boundary

[Nidish96]

Using the code from python-cell (https://github.com/twmr/python-cell.el ) , I've added a new face for highlighting the active cell (see screenshot below) . Other than this, I've also expanded the defaults for the boundary regexp to include those of outshine mode. This way it's quite streamlined. Have also added "*" to each subheading in the code-cells.el file so this also makes use of code-cells and outshine.