Add option --milestone to open_issue command

Closes #86, wanted to get an opinion on this before I finished up the feature with tests.
I agree it is a convoluted.

The code I have actually won't work because it runs the conditional only if the milestone is nil.

What do you think about something like this?

```ruby
# If the a milestone is an option, make sure the milestone is on GitHub
if @issue[:options][:milestone]
  unless milestone_exists_on_github?(self.client, "#{@organization}/#{repo_name}", @issue[:options][:milestone])
    puts " --> Milestone not found, skipping '#{repo_name}'"
    next
  end
end

def milestone_exists_on_github?(client, repo, milestone)
  !client.milestone(repo, milestone).nil?
end
```