Feature Request: Add indicator that a cell has been modified since it has been last executed

[ehsteve]

It is sometimes not clear whether a cell has been changed and therefore whether the code and the output are in sync. An easy way to do this might be to highlight the cell a certain color when it is edited and remove the highlight when it is executed.

[JamiesHQ]

@gnestor this is a nice idea = )

[gnestor]

@ehsteve Care to submit a proof-of-concept PR? Let me know if you have any questions along the way.