Add labels for mixing ratio lines and adiabats on skewT

[ssalesky]

It would be helpful to have the ability to label mixing ratio lines and perhaps also dry and moist adiabats. Maybe I missed it in the documentation, but I haven't found an easy way to do this.

What I have in mind is similar to the examples below:
Stull, Practical Meteorology
http://bretwhissel.net/skewt/skewt-loc.pdf

[dopplershift]

Definitely a good idea. Would probably be readily do-able if/when we make these proper gridlines (or grline-like) as suggested in #112.

[dopplershift]

Also, did you see our user survey?

[markmuetz]

This would be good - the plots would be a lot more useful if all the lines were labelled.

[jrleeman]

Thanks for the vote @markmuetz - we'll try to get this bumped up the stack and get #112 closed out.

[brian-rose]

Hi metpy team, just dropping by here to give a +1 to this issue! Teaching thermodynamics right now, and I'm currently struggling with putting useful labels on mixing lines. Unlike the dry and moist adiabats, I can't easily infer mixing ratio values from the horizontal temperature axis.

[dopplershift]

Thanks @brian-rose ! Would love to have your feedback on the interface presented in #2488, which provides a declarative style plotting interface (inline with our existing "simplified" plotting interface) and features some support for labelling.

The challenge (that I may have to compromise on) is finding a way to do this that integrates within Matplotlib's gridline/ticking machinery rather than using some kind of judicious hackworkaround.