Support Preview Pane for Block Math

[benrbray]

Resolves #28.

• Display math now shows a preview pane. Inline math previews may be supported in the future.
• It should be fairly simple to adjust the appearance of the preview pane with CSS.
• Added an initialText parameter to insertMathCmd

Breaking Changes

• Added new isBlockMath argument to MathView constructor
• mathPlugin and mathPluginSpec are now functions that accept an options parameter
• The mathPlugin state now has an extra flag indicating whether previews are enabled

TODOs:

• Investigate whether inline previews can be trivially added (e.g. via CSS) or whether they should be added in a separate PR.
• Update the docs site with a checkbox to enable/disable previews?
• Better error reporting when preview is active?

[Saul-Mirone]

I think it's better to have the inline previews in a separate PR, since there's no common things with block preview IMO.