Switch CDN for MathJax

Fixed #5 by switching to CloudFlare CDN from the existing MathJax CDN which will be shut down.

Restore compatibility with Firefox via WebExtensions

Fixed #4 by falling back to the local storage API when the sync storage area is not available.

Customisable delimiters

Added toggling of different delimiters and an option to specify custom delimiters.

More general injection points

This should fix problems that arose from the LaTeX not being un-rendered upon merging of items and copy-pasting.