fix(plugins): if $HOME/.ccw does not exist, silently bypass plugins
feat(lein): Activation of the Generic Lein task launch popup
feat(lein): enhancements of generic lein launch ergonomics

- prevent early submit of the popup
- works with or without a project
doc(ChangeLog): prepare Changelog for 0.21.0
chore(version): Update version number to 0.21.0