Support styling classes

[joostas]

ClassDef and class doesn't change style of elements. See example below:

graph TD A[Christmas] -->|Get money| B(Go shopping) B --> C{Let me think} C -->|One| D[Laptop] C -->|Two| E[iPhone] C -->|Three| F[Car]

style A fill:#f00 classDef def fill: #f00 class B def

Style changes fill color of A element. But assignment of class def to element B doesn't change fill color of B.

[tylerlong]

I confirm that classDef of mermaid 7.1.0 is broken.

[tylerlong]

Fixed.