markLine label missing rotate option

### What problem does this feature solve?
Much like markArea and markPoint labels, markLine should likely allow for the ability to rotate labels. Label rotation and manipulation is important in the event several markAreas and markLines are used together with labels as labels can often overlap when drawn dynamically.

An example of this would be graphically displaying inventory level data along with key metrics where key metrics are represented via markLines and markAreas. These markLines and markAreas have the tendency to overlap as inventory exhaustion and replenishment can occur near similar date ranges throughout a given sales year. Being able to rotate and reposition markLine labels would help circumvent the frequency of label overlap.

### What does the proposed API look like?
Extend the current markLine label options to also include the ability for label rotation. According to current documentation, it is worth noting that many of the options available to markPoint and markArea are not currently implemented for markLine labels. Presumably, label functionality for markLine, markArea, and markPoint should be similar or mostly identical (which appears to be the case with markPoint and markArea label options).