Add an admin page for module parameters

Problem/Motivation

Many of the hardcoded parameters in the module should be choices to the site administrator.

Proposed resolution

Add an admin page and expose module parameters for the site administrator.

Remaining tasks

User interface changes

An admin page at /admin/config/services/markdownify that exposes parameters and settings as switchable options.
Actual parameters to be determined, but must include choice for /node/N.md and /[path:alias].md

API changes

None expected.

Data model changes

None really, except for making parameters and settings dynamic.