Add cssText() method to adaptor and handle dynamic rules in HTML output.

[dpvc]

It turns out that when a stylesheet has dynamically added rules (like the ones MathJax produced for CHTML output when adaptiveCSS is true), those rules are not returned by innerHTML, outerHTML, or textContent. This makes it hard to obtain the CSS needed for HTML output in that case.

The PR adds a cssText() method to the adaptor class so that these rules will be included in the output. This can be used in place of innerHTML or textContent to get the list of rules when adaptiveCSS is being used.

[dpvc]

The alternative would be to modify innerHTML, outerHTML, and textContent to include them, but that makes these function less compatible with what the browser actually does, and lead to confusion if these were used on a node containing a style node, like the head node, where the normal HTML behavior would still occur.

[zorkow]

One typo.