gemgen v0.5.0

This version adds a new flag for horizontal rules (-r, --horizontal-rule).
You can now print them using any string you like, including newlines to
create a codeblock for example.

The default horizontal rule was changed to a blank string (hence the
version bump). Having the 80 character wide - as a default was not ideal
for screenreaders. The new default is ~~~ which should be more friendly.