Adds WpCliMissingCommand and tests.
Adds missing command to factory
Improves error message to include cwd.
Adds ability to use missing command if wp-cli absent.

Also works if `wp cli cmd-dump` returned empty list.
Merges branch 'feature/improve-error-messaging-for-wp-cli' into develop
Bumps version to 0.1.15.
Build 0.1.15
Updates changelog for 0.1.15
Merges branch 'release/0.1.15' into develop