User-Definable Substitution Patterns

Issue Summary

On a site we are using a customized version of BBCode; it is customized because adding new substitution patterns to BBCode involves modifying code, and the modifications that we have brought are only useful to our site. The problem, of course, is that every time BBCode gets updated, we have to migrate code, test, etc.

It would be best if the substitution patterns could be stored in a database and created/re-ordered through an administrative interface (keeping in mind that some transformations depend on others).

Also, it would be ideal if callback-style substitution patterns could be registered with BBCode from an outside module without having to modify the code; I'll open a new request for this.