Make the scaffold command conditionally dependent on WP_Filesystem
Allow community packages to use WP-CLI testing framework

First pass
Handle community package context where these files are copied
Travis file for package tests
Helper utility for building the test config
Commit the helper utility
Persist a supplied config path variable
Bash script for installing package tests
Make sure the bin directory is created
Update the docs some
Ignore `config.yml`, which means developer has cloned into `~/.wp-cli`
Test explicitly for composer.json file
Ensure `install-package-tests.sh` ends up as executable
Merge pull request #1309 from wp-cli/scaffold-package-tests-2

Add `wp scaffold package-tests` command