Add `tea comment` and show comments of issues/pulls #313

Add tea comment <issue/pull index> [<body>] (partial fix to #172)
- I like tea comment / tea c better than tea issue comment, but I'm open for suggestions
Add a prompt to tea pr <idx> if comments should be shown.
Add a prompt to tea issue <idx> if comments should be shown.
Add a --comments flag to avoid prompting (i.e. keep a programmatic mode).
- if flag is unset, will prompt
- if flag is set, will not prompt
- if output is piped, will not prompt

this is ready now (comment made with tea :D)

Please resolve the conflicts.