Markdown UML & flowchart support

Add UML sequence and flowchart support to Gitlab Markdown editor

Alice->Bob: Hello Bob, how are you?
Note right of Bob: Bob thinks
Bob-->Alice: I am good thanks!

And flow charts like this:

st=>start: Start
e=>end
op=>operation: My Operation
cond=>condition: Yes or No?

st->op->cond
cond(yes)->e
cond(no)->op