Custom rules #41

This would allow people to create their own rules, likely requires #12 to be implemented first.

Although this could theoretically be done before, it would require creating a custom system, which would likely have to be replaced once #12 is implemented. This has a few possible problems:

1. One of the main blockers for Rewrite in a structured manner #12 is knowing how to create a proper system for organizing all the rules. Figuring this out likely requires much of Rewrite in a structured manner #12 to be implemented anyways.
2. If the custom system is just hacked together (or just using plain javascript eval), it would likely result in the user having to rewrite all of their rules, unless they can somehow be migrated to the new format (virtually impossible in the case of javascript eval).
3. Plain javascript eval is not only highly sensitive to changes within the userscript (which doesn't have a stable abi), it's also blocked on a number of major websites, meaning that custom rules will not work on them if using the userscript.

Whatever solution is implemented for this, it will take a significant amount of time to implement and document. Users will have to learn a new system, and changing the system midway through is going to hamper user experience (setting aside the issue of a relatively large amount of duplicated development time).

As unoptimal as it is, until #12 is implemented, for now the best option to add your own rules is to submit an issue (or contact me directly) for it to be included upstream, then edit the script locally until then (or use git master).