Move key binding to the main file
Rename `display_message` -> `echo_message`
Plugin installation does not perform update

Plugin installation just installs new plugins.
Already installed plugins aren't updated.
Rename script for plugin installation
U key binding shows a list of installed plugins
update_plugin script
Update plugin installation messages
Change plugin update wording
Update readme with new functionality
Update plugin installation tests
Enhance plugin installation test
Add tests for updating plugins
v1.0.0