Initial content
Plugin installation notification tweaks, test fixes
Replace echo-heredoc with cat-heredoc
Use trace headers where applicable
Set plugins-install to install from GitHub via HTTPS rather than SSH

SSH requires Plushu to have a public key on a GitHub account,
which is unlikely.
HTTPS paths start with slashes, not colons
Make PLUSHU_PLUGIN_NAME available to install scripts
Fix ssh url match in plugins:install
Trim definition name more effectively
Handle HTTP(S) URLs correctly in plugins/install
Supply PLUSHU_PLUGIN_NAME to reinstall and uninstall as well
`plushu plugins`: List one plugin per line
Give Plushu user ownership of newly-installed plugins as root