Add define command to define common words and terms

[ScottMansfield]

Add a simple command to define a word. I was thinking this should be an API somewhere we can hit to get a reasonable answer but realistically it will have to be a curated list.

The terms to be defined would probably just be Go terms, but there could be a fallback to a dictionary API for ESL people. It's probably over-complicating though.

Previous discussion here: gobridge/gopher#33

[theckman]

@ScottMansfield are you aware of anything that exists like this today, even in a text form?

[theckman]

This is basically finished, but am thinking it makes sense to get more definitions in before closing this:

https://github.com/gobridge/gopherbot/blob/master/glossary/terms.go#L5

Mind contributing some?