Images/Attachments are only generated into the primary alt folder and
not into the secondary alt. So image url need to get resolved using the
primary alt.

Lektor mistune markdown is not doing any link resolution. Let have
similar behavior.

Alot of modifications was required to make use of `url_to()` during
rendering of the markdown. This was required to make sure we have a
context when executing the rendering.

Fix the plugin description to replace the old description imported from
asciidoc.

Following recommendation made by nixjdm, make the plugin configurable
using a config file. Load configuration for the engine, extensions to be
enabled and also load their respective configuration.

Add note to the README file about incompatibility with `markdown-*`
events and any plugin build around them.

Replace all the call to asciidoc by a call to python-markdown.