Add links to source, docs and issues to `composer.json`.
Add links to source, docs and issues to `composer.json`.

These are programmatically scanned in several places (like packagist.org) to provide direct access to these resources.

trepmal/wp-cli,trepmal/wp-cli,gitlost/wp-cli,miya0001/wp-cli,2ndkauboy/wp-cli,2ndkauboy/wp-cli,rodrigoprimo/wp-cli,GaryJones/wp-cli,miya0001/wp-cli,rodrigoprimo/wp-cli,gitlost/wp-cli,rodrigoprimo/wp-cli,aaemnnosttv/wp-cli,aaemnnosttv/wp-cli,gitlost/wp-cli,aaemnnosttv/wp-cli,2ndkauboy/wp-cli,GaryJones/wp-cli,trepmal/wp-cli,GaryJones/wp-cli,miya0001/wp-cli