Allow users to see pull request comments

Modify 'show' to accept an optional '--comments' flag.
When comments flag is specified, 'show' will include pull request comments (in-line comments that are part of the pull request), issue comments (comments attached to the pull request but not to a specific line of code), and commit comments (in-line comments that are part of the commit).
Comment API requests can be slow, so are not included by default.
Comment data can get large, and is not cached.