Add option to fold repeated activities in sequence diagrams

[bkryza]

Currently, any function or method called multiple times in the span of a single sequence diagram is rendered fully every time it's called, which can generate very large diagrams for some code bases.

There should be an option to enable folding of repeated activities, so that any activity is rendered fully only once and then only a call to that activity is rendered but folded with some indicator...

Might be relevant for #316