Add arrow-text attribute to symbols

[josephtocci]

For my flowchart I had to make a change and label a specific arrow on a operation block. These are my changes.

[adrai]

I'm merging right now... but can you add here a comment on this PR for documentation? So others can see your example?

[adrai]

v1.9.0

[josephtocci]

There is a particular step in my process where there is only one path, but sometimes that path is temporarily closed and it is best represented with a note on the arrow instead of a decision block.

Not my actual example, but this is how I put the text on the arrow:

'flowstate' : {
    'example' : {'arrow-text' : 'Need to wait for other process to finish before proceeding' }
}

[FavorMylikes]

Can I use it on parallel at different path?