Support printing enhanced graphs in Tikz

[danielhers]

With enhanced=1, the enhanced graph will be printed under the text. Example:

[martinpopel]

Nice. Thanks. I have other ideas, but no time to implement them (noting it here just in case anyone finds it useful):

• Hide the enhanced edges that are included also in the basic dependencies graph (or allow different colors for the edges which are "extra" or "missing" as in examples on the UD page.
• Allow the as_tree mode by drawing a "minimum spanning tree" (edge weight could be 0 if the edge is included in basic deps and 1 otherwise) and the remaining edges would be visualized as non-tree edges.
• Implement this also in write.TextModeTrees.