Hybrid doc and result column

[tlunet]

Would be useful to be able to add either plot or markdown stage in the results stages (right column, formerly plot stages) and also in the docs stages (left column).

[MichelKrispin]

Currently, the docs stage can just use a different title such as 'Result' and then display some stuff while the PlotsStage can use the caption with having the plot set to None (see #23).
But I'll leave this issue open as it's not a very clean solution right now.