loader, plugin, rules: add rules_match() decorator

[Exirel]

Description

Fix #462 with the last option: have a rules_match (name suggested by @dgw) decorator with a flag for RAW/PLAIN/BOTH.

• add a new decorator sopel.plugin.rules_match
• add a flag enum sopel.formatting.MatchText: the enum is required in various modules (plugin, plugins.rules, loader, etc.) and it fits the theme of the plain function
• add a note in plugin documentation about how rules are using the raw version by default
• updated AbstractRule.parse from getting only text to text, plain, making it a bit awkward to use
• base Rule and SearchRule were easy to deal with; FindRule required a bit of trickery, which means that you can't partially match both version, if you find anything in one, the other is ignored (when using the match BOTH)

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches

[Exirel]

There was a conflict and no review at the moment so I took the chance, rebased, and now I'll ask review. 😛

[dgw]

Haven't looked at tests yet, since I asked to change some names and those will change the related tests too if you accept the ideas.

You can ignore my lament about the direction this took vs. what was initially planned in #462, if you want. I don't quite understand all the pain points related to find and search, but I suspect at least the lazy ones have to do with "oops, we already shipped this public interface that would have to change" and........ yeah, OK. That would be a pain, yes. And if not all places where rules can be defined could support a new kwarg, then it doesn't make sense to be inconsistent.

[dgw]

Letting @Exirel take the last few steps here, but per discussion in this thread + on IRC, we are postponing this feature until the next version in favor of reworking some innards to make it (and other ideas like it) work better. The current plan is to close this PR, and build a new one on top of it later.