pull show: Show both regular and code comments

Fixes #83 and also retrieve labels for pull requests.
issue show: Improve issue formatting
issue show: Add --summary option

With this option, the issues are printed with the same format as the
`issue list` command.
issue show: Retrieve comments only if needed
pull show: Print review comments correctly

Review comments belonging to the same piece of code is now printed in
a group, leaded by the diff hunk that's being commented. A group of
review comments and regular issue comments are now separated by a more
graphical horizontal bar.