Ensure MathQuill.TextField's are .mathquill-textbox

https://github.com/mathquill/mathquill/commit/f4ce7e9647cb2d3d05ff47e0551a7ed7ee258057#commitcomment-6262646
Match wording of README and publicapi.js comments #docs

In particular, consistently say "HTML element" everywhere, instead of
sometimes also saying "DOM element".

Also, I originally wrote multiple variants of the global MathQuill() fn
comment, meaning to delete all but one, but left 2, so combine them.
And copy over explanation that identity of API object is guaranteed.
Fix MathQuill::revert(), test with README example
Fix and test passing null/undefined to global API fns

Include tests for every README example
Don't use .mathquill-embedded-latex for styling

Was being used to style inner math fields (\MathQuillMathField{}), but
want .mathquill-embedded-latex to be entirely external. Now its only
mention internally is on document ready, find all
.mathquill-embedded-latex and render them as MathQuill.StaticMath().

Styling should be done with only internal CSS classes like
.mathquill-rendered-math (which really should be .mq- not .mathquill-).
Rename .mathquill-{embedded-latex => static-math}

Matching the name MathQuill.StaticMath().

This one's easy because it was already basically only used externally.

Recommend --word-diff-regex='[a-z-]+|.' to view diff.
Rename external .mathquill-{editable => math-field}

Styling should be only done with internal CSS classes, so now the README
refers to .mathquill-math-field and never .mathquill-editable (which
should be .mq- not .mathquill-).

The new .mathquill-math-field CSS class is entirely external, its only
mention internally is on document ready, find them all and render them
as MathQuill.MathField().

Recommend --word-diff-regex='[a-z-]+|.' to view diff.
Rename external .mathquill-{textbox => text-field}

Styling should be only done with internal CSS classes, so now the README
refers to .mathquill-text-field and never .mathquill-textbox (which
should be .mq- not .mathquill-).

The new .mathquill-text-field CSS class is entirely external, its only
mention internally is on document ready, find them all and render them
as MathQuill.TextField().

Recommend --word-diff-regex='[a-z-]+|.' to view diff.
Rename .mathquill-editable to .mq-editable-field

It's an internal CSS class used for styling, and this matches the name
EditableField.

Recommend --word-diff-regex='[a-z-]+|.' to view diff.
Unify .mathquill-textbox and .mq-text as .mq-text-mode

.mq-text is just straight up replaced with .mq-text-mode (named after
TeX's text mode, as opposed to math mode).

.mathquill-textbox is replaced with .mq-editable-field.mq-text-mode
("an editable field in text mode"), so once .mathquill-rendered-math
becomes .mq-math-mode, TextField's will be symmetrical with MathField's,
which will be .mq-editable-field.mq-math-mode.

We don't have to worry about selectors for TextBlock's (.mq-text) now
accidentally selecting TextField containers, because those selectors
are always nested inside .mathquill-rendered-math's.

Recommend --word-diff-regex='[a-z-]+|.' to view diff.
Rename .mathquill-rendered-math to .mq-math-mode

Named after TeX's math mode, symmetrical with .mq-text-mode.

Nicely enough, makes the TextField constructor even more symmetrical
with the MathField constructor.

Recommend --word-diff-regex='[a-z-]+|.' to view diff.
Rename .{mathquill => mq}-root-block

For consistency with all of MathQuill's other CSS classes
Don't auto-MathQuill-ify on document ready

So many benefits:
- multiple versions/builds of MathQuill can be included on the same page
  without them fighting each other to the death
- API consumer doesn't have to worry about waiting for MathQuill's ready
  event handler to do its thing or whether it already has
- simpler to describe in docs, just "do this" rather than "at this
  ill-specified point in time this will happen"
- no magic 'mathquill-' CSS class names
- less code

Also rewrite much of Usage in README to be more readable overall, in
addition to/along with updating it to require the API consumer to call
the MathQuill constructors.
Merge pull request #308 from mathquill/api.no-auto-mathquill-ify

API change: Don't auto-mathquill-ify on document ready