Added Short ID Field Label and Type

Added Short ID to the list of available Field Labels and Types. One potential use for such a field, when creating a website, would be to perform your own homegrown URL shortening, rather than using a third-party service.

Added New Export Option to Concatenate HTML or Markdown

The new options, described on the Import and Export Your Notes page, will write out a single file for an entire Collection, concatenating all of the Notes in the Collection.

Fixed a Footnote Bug

Footnotes were not being properly handled when they contained an automatic link (contained within angle brackets). This has now been corrected.