Instead of a two-step process handled by the application
(`Issue::new_comment()` and `Comment::set_description()` or
`Comment::edit_description()`), make a simpler-to-use single-step
`Issue::add_comment()`.
Move the implementation details from Issue to Comment.
Better log message when adding a comment.
This starts cleaning up the Issue API.
* Start separating the public API from the internal API.
* Make `Issue::new()` and `Issue::edit_description()` better behaved,
simpler, reduce code duplication, and also produce better git log
messages.
* Update `ent` to call the changed `new()` function.
* Add some comments documenting the Issue API.
* `ent new` and `ent edit` now use the editor specified by the EDITOR
environment variable, if any. Defaults to `vi` if unspecified.
