Add graph titles

[haffi96]

• This is to add a graph/image title
• Works on .er, .dot, .jpg and .img

Examples: .er:

.dot

[haffi96]

Hi, just wanted to open this PR to show a demo of the feature. Will update unit tests if the feature is a good idea/accepted

[maurerle]

Hi @haffi96 - I like the feature - would be cool, if you add this as an example and add some unit tests

Thank you very much!

[maurerle]

I just fixed the behavior and added the title for mermaid and mermaid_er output, but did not yet add tests and documentation :)