* Refactor formatting logic to better calculate needed padding between
issues and sections.
* Add `list contexts` command to list all known contexts according to
the contexts configuration and the contexts defined in issues.
* Be more intentional about when the default context is used. Don't
override existing context values in issues when changing their state.
* `edit` now allows multiple issues to be edited.
* Change single-issue display to be more verbose, listing all the
properties and tags on an issue.