v2.3.0

Makes the user aware of any new, not-already-manually-approved extensions
that the source markdown wants to load.

Also checks all loaded extensions for a user_warnings function that returns
a list of messages to display to the user before using the extension.

Adds new -i, --safe, and --no-ext-warn command-line arguments.

type	ticket	description
feature	#109	More robust extension handling, does not auto-load by default