Lazy extension update

[dpvc]

This PR updates the lazy-typesetting extension to add several new features.

• The ability to typeset all untypeset math on the page before printing it (so that it will print properly, even when parts of the page haven't been viewed). (ui/lazy and printing MathJax#2711)
• A new option to control the margin to use for the observer trigger (so that expressions slightly outside the viewport will be typeset in anticipation of scrolling.
• The ability to mark some containers to force the math they contain to always be typeset, regardless of whether they are in the viewport. This allows math that is used in JSXgraph, for example, to be placed properly. (Lazy Typesetting vs. JSXGraph MathJax#2720)

There are also a typo in a comment that is fixed, and a redundant definition of rerender() in the MathDocument interface declaration was removed.

The three bullet points are each done in a separate commit, so it might be easier to look at them separately, rather than mixed together, as in this PR. The third is the most complicated, as there was considerably infrastructure needed to manage that. The new lazyAlwaysTypeset option is either an array of container nodes whose contents should always be typeset (non-lazily), or a single string or array of strings that represent CSS selectors for the containers to use. E.g.,

    lazyAlwaysTypeset: '.always-typeset'

would make all containers with class="always-typeset" to have their math typeset regardless of whether they appear in the viewport or not.

[zorkow]

Just minor issues. O/w lgtm.

[zorkow]

lgtm.