Support 'trunk' and 'nightly' version arguments for wp core download.
Support 'trunk' and 'nightly' version arguments for wp core download.

- Modifies Core_Command::get_download_url() to interpret 'trunk' and 'nightly' versions
- Core_Command::get_download_url() now conforms to WordPress coding standards
- Adds support for downloading and extracting .zip files as nightly builds are own available in zip format.
- wp core download --version=trunk is only supported if ZipArchive is present.

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