Support for parsing HTML-like labels

[soerface]

Using HTML as a label is actually valid in graphviz, try copying this example into edotor: https://stackoverflow.com/a/53464014/458274

Syntaxhighlighting will fail on tag attributes, closing tags.

Feature and grammar is described here: https://graphviz.gitlab.io/_pages/doc/info/shapes.html#html

[nikeee]

HTML identifiers are actually implemented. However, it seems that it is not implemented according to the spec.

Also, I'm not sure if ports are currently supported. At least they aren't supported for auto completion.