WIP: Find command

Implements a find command (cf. #146)

Still work in progress, but in case someone wants to try it out and has some (much appreciated) feedback or bug reports, here it is.
Type `ghi help find` to see how it works.

Some issues remain, especially with pagination. Currently not sure if I don't see it or if the pagination of github's search API is a little broken. We'll see.

Left to do:
- Cleaning up code
- Probably some more options (milestones)

In #146 I mentioned adding an option to use this new command indirectly in the `list` command. I'd be less inclined to do that now, I guess it's not worth the trouble because there are some issues involved - the API's are a little different after all.

I think it's quite a useful feature in the end.