Better math mode

[kmgb]

Currently, math mode just makes all text italic by default.
This doesn't make sense for some characters with italic variants, like capital Greek characters, which LaTeX renders as non-italic even in math mode.

Math mode should generally support:

• Only converting Latin and lowercase Greek characters to italic variants
• \sin, \cos, \log, etc to override this italic behavior
• (potentially out of scope) different space handling -- ignoring by default, better spacing around operators, etc.