$x_y$ should not make the rest of the line italic

[cben]

Needs changes in CM's markdown mode:

1. Recognize $...$ and don't parse stuff like _ and * inside it. (a-la backticks ...)
2. Adopt pandoc-like rule where _ and * inside a word don't count. Not a full solution but helpful in any case.

[cben]

Another side effect of this problem: $a<b$ breaks syntax highlight of everything after that because markdown mode treats it as start of <b ...> tag and switches to html parser. Workaround: $a < b$ (with space after <) works fine.