more sophisticated line breaking in wdl-format

[a-frantz]

#242 introduces a MaxLineLength configuration for wdl-format. This is a great improvement, but leaves some more to be desired. It leads to some awkward line breaks. This can be improved by making the algorithm a little more sophisticated.

Potential improvements:

• a priority list of potential line breaks for each line we can backtrack through and select "the most desirable" token to line break on.
• a stack for "matching tokens" that should be line broken in tandem (open/close parenthesis/brackets etc.)