Add examples and documentation for plugin configuration

[oz]

@dominikh How can we help? Do you have any wishes regarding some examples that could be written?

[dominikh]

Well, right now, there is zero human-language documentation on how plugins are written, how matchers work, what can be configured etc. All we have is a very brief plugin example in the "Getting started" document (which incidentally links to a "coming soon" plugin document...)

So what's really needed is a guide-like description of how the whole thing works. Examples by itself aren't really needed, the examples/ directory is full of those.

[wyattisimo]

In reference to issue #47 regarding plugin prefixes, the documentation here (http://rubydoc.info/gems/cinch/Cinch/Plugin/ClassMethods:match) is not clear on this point.

It lists the prefix default as nil and describes the attribute as "A prefix overwriting the per-plugin prefix." But this is misleading because it does not overwrite the per-plugin prefix, rather it creates a per-plugin prefix by overwriting the plugins prefix specified in bot options.

I spend quite a while in utter confusion over why my plugin's execute method was not firing. I thought I was using a nil prefix until I finally discovered that bot options globally defaults the plugins prefix to /^!/.