Support verbatim multiline text in node labels

[cdmihai]

As far as I can tell the only way to have multiple lines of text in a node is by adding <br> tags:

graph TD
    B[Line 1<br>Line 2]

However, this gets really tedious for larger text, and I'd really like if we could do this:

graph TD
    B[
        Line 1
        Line 2
        ...
        Line n
     ]