Don't style all math as tex math delimiters with defineMathMode().

(Still broken in that it's not colored according to context;
but at least it's not all colored purple.)
Show display math centered on a separate line.
Works right whether formula is in middle of text or on its own line in source.
Closes #16.

Changes styling: .math -> .inline_math, .display_math

Credit for discovering the magic `width: 100%` method goes to https://github.com/rhythmus/mathdown/commit/e548ce987588b1ec1e5244ada0f3b6b97b71ba3d