Ongoing build / template process improvements

[snostorm]

This carries my wish list from #257

Open questions:

• What dependencies in the package.json are no longer needed - thanks @arthurvr via Remove unnecessary dependencies in package.json #279

High Level:

• Proof-of-concept metadata tracking around using alternative templates on certain articles
• Tracking a manifest of what articles are to be created (for i18n cross-linking.)
• Proof-of-concept page that is not markdown based. Example: the homepage shouldn't really be a markdown file, but I may make a new page (like downloads/ release history/ etc.) as a test.
• Live reload integration

Changes:

• The "templates" task will now parse .html files per-language, not just Markdown
• The English index.md switches .html, uses a dynamic list of downloads
  • partially done, see home.html proof of concept
• Adds Handlebar "helper" {{i18n ...}} for looking up dynamic or (if desired) static key
• Adds Handlebar "helper" {{hb ...}} for parsing i18n strings with nested variables
• Adds Handlebar "helper {{link ...}} for easy i18n-friendly links, or more manual links

Merged:

• i18n deep merge (from template.json files against English defaults)
• Switches source/project.json to source/project.js to allow dynamic "stuff" like download link lists

Example of nested variables (template.json)

{
  "verbose_version": "Version {{project.current_version}}",
  "home_download_links": "Download for {{> current_download_links}}"
}

[snostorm]

I'll post the next PR which adds some of the "Changes" section of to dos once #269 lands.

[snostorm]

#276 adds most of what was discussed under "Changes"