Allow \middle to work when a style or color is open. (mathjax/MathJax#2663)

[dpvc]

The current implementation for \middle only works when there are no open style or color changes, and so it fails with

\left( \color{red} x \middle| y \right)

for example. This PR introduces a MiddleItem stack item that acts as a close item, so it will close any pending and style items. The LeftItem looks for a MiddleItem and processes it (by adding the needed mo) without closing itself so that it will remain open until the following RightItem occurs.

In addition, the \middle and \right delimiters now save the color that they should be in, since \left(\color{red}x\right) colors the right brace red in actual LaTeX (and similarly for \middle), and apply those properly (they didn't before).

Finally, I made the LeftRight constructor take the actual delimiter (and color) as an argument so that you don't have to set it after it is created.

Resolves issue mathjax/MathJax#2663.

[zorkow]

lgtm.