Support gist

[owenthereal]

$ gh gist -d "awesome" file1.go file2.rb

$ gh gist file1.go file2.rb
# open editor for description

$ gh gist file.go
# file.go doesn't exist
# open editor for edit

We could do better than this.

[dgryski]

API docs: http://developer.github.com/v3/gists/

[owenthereal]

create gist in #165