Allow `data-background` images to load even if there is whitespace at…

Fixes #2032.

Allow JS files with query strings to be loaded.

Fixes #1944.

Block F5 in speaker notes window, avoid disconnects

Ignore lock file of newer node versions

Allow tex2jax options to be passed into math plugin.

Closes #2026.

This does not change the default behavior of the math plugin,
but it allows $ delimiters to be disabled (which is usually
what you want, since otherwise normal uses of $ for currency get
treated as math delimiters).

To use:

    Reveal.initialize({
      math: {
        tex2jax: { inlineMath: [['\\(','\\)']],
                   skipTags: ['script','noscript','style',
                              'textarea','pre'] }
            },
      etc.
      });

adds note about necessary markdown scripts to README

Update README.md

corrected minor typo in readme file

Export isSpeakerNotes

Register search key shortcut for help overlay