Feature request

\underline is a valid LaTeX command but is not available in mathtext as documented herehttps://matplotlib.org/users/mathtext.html#mathtext-tutorial. My guess is that this is not common in math formulas but I have a user that does have a need for it in their formulas. I realize I can doplt.rc('text', usetex=True) to pass off the entirety of LaTeX rendering but it would be nice to have this available within matplotlib. Any arguments for/against?

Matplotlib version

• Operating system: Windows 10
• Matplotlib version: 1.4.3
• Matplotlib backend (print(matplotlib.get_backend())): TkAgg
• Python version: 2.7.13
• Jupyter version (if applicable):
• Other libraries: