Remove .is{First,Last}Letter flags from Letter's

Also remove Letter::latex() entirely. Instead of overriding
Symbol::latex() (which just outputs .ctrlSeq), leave as-is and have
Letter::autoUnItalicize() directly modify .ctrlSeq, using .letter as
saved by Letter::init() and never modified again.

Prep for latex output being more complicated than just two mutually
exclusive flags (i.e., should be outputting \operatorname{...}
sometimes).
Clarify Letter:autoUnItalicize #cleanup

Setting .ctrlSeq and the CSS classes is about to get more complicated,
so separate into its own section, and determine `last` at the beginning
before doing any of it as soon as we know we've found an unitalicized
word (except setting each `un-italicized` CSS class, simplest to do that
in the same loop where we find `last`).
Fix UnItalicized LaTeX output to use \operatorname{...}

...when outputting UnItalicized's that aren't built-in LaTeX commands,
like arcsec. Also parse \operatorname{...} stupidly, by pretending the
letters inside are just variables and auto-unitalicizing as appropriate.

I was originally going to have 'inj' and 'proj' auto-unitalicize as
\operatorname{inj} and \operatorname{proj}, too, but it was a ton of
code (the cleanup in the previous commit was pretty important in making
the diff readable, though it's helpful even now) and I eventually
realized this was much simpler and I should err on the side of fewer
3-letter auto-unitalicized commands anway. So now, 'inj' and 'proj' no
longer auto-unitalicize at all, only 'injlim' and 'projlim' as a whole
get auto-unitalicized. This is implemented as a special case, but
whatever.

This problem was brought up in #286. Also fixes #289 (including [1] by
only setting a 'last' CSS class but no 'first' class in the middle of
two-word operator names; choice of that over 'first' but no 'last' was
largely arbitrary, mainly it just looked more right to see extra space
after rather than before a word).

[1]: https://github.com/mathquill/mathquill/issues/289#issuecomment-41447574
Fix #292 support arcsinh etc as well as arsinh etc
Fix #300: \inf, \liminf broken

Ugh at first I thought this would be as trivial as redefining the florin
to descend from Letter rather only Symbol, but then I realized the
florin character looks italic when what we want is roman 'f', so had to
refactor that out, then found that \inf was an incorrect alias to
\infty! Idiot. So, I hope no one using MathQuill has been relying on
\inf, 'cos this totally changes the meaning of that.
Simplify MathQuill.addAutoCommands() args parsing #cleanup

https://github.com/mathquill/mathquill/commit/b2bfcd8a928be07b6736fe04be3052bb4584bdf2#commitcomment-6163517
Fix #286: add MathQuill.overrideAutoUnitalicized()

https://github.com/mathquill/mathquill/issues/286#issuecomment-40857307
Merge pull request #301 from mathquill/fix.auto-unitalicized

Fix auto-unitalicized