gemgen v0.3.0

This version adds:
 - concurrent file reading/writing
 - options for link handling in paragraphs and headers
 - option to print or not print blank lines after headings

The command line options were slightly changed to allow for these
features and to make room for future plans. There are now long and short
versions of each flag and the `-E` option has been removed. Instead you
can use `-e off`, `-e markdown`, or `-e unicode`.

The unicode option is currently undocumented. I don't want to carelessly
encourage its usage which would harm screenreaders. Those who know about
the feature should only use it when they can provide a alternative
accessible version of their documents.