Add support for Flow Notes

[SlavaVedernikov]

Context

Sometimes it's useful to have Notes rendered in Sequence diagrams for some lines/arrows between participants to give context or to explain some details of a particular interaction etc.

There is a limited support for Notes in the framework already, but it needs more attention.

Requirements

Add support for Notes for Flows, so that those notes can then be rendered in Sequence diagrams as follows:

note left
This is a sample note.
It can also have multiple lines
end note