v1.4.0: health subcommand, Loki log forwarding client

This release includes two new features:

• The new pebble health subcommand, implemented by @rebornplusplus in #299.
• Log forwarding to Loki, implemented by @barrettj12 in #267.

In addition, we improved the error message from exec when the $HOME directory doesn't exist, and other minor changes.

Full Changelog: v1.3.0...v1.4.0