Add list contexts, refactor display logics.

* 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.

src/pitpkg/private/version.nim

@@ -1 +1 @@
-const PIT_VERSION = "4.1.0"
+const PIT_VERSION = "4.2.0"