Multi option evolved

[sullivanpt]

This PR includes and builds upon @chunksnbits super cool PR #6. It makes the following additional changes:

• the plugin options is changed back to an object so a global theme can be specified.
• the multi-options array form is now an attribute bundles within the options object. As before, bundles can be a single option, or an array of options one per output file.
• the multi-options output files can be built dynamically by providing a function for bundles.
• page generation is paused until after the webpack dev compiler finishes
• the generate page URL is fixed to be Windows compatible
• the 5 second inter-generate page delay is removed
• the README is updated
• package-lock.json updated (mostly so I can build directly from my fork)

If maintainers prefer I can split these features up into multiple PRs (although there is some difficulty as the original PR renamed several internal variables). Thanks!

[sullivanpt]

Replaced with PR #21 which has package-lock.json removed.