Add `WP_CLI_CUSTOM_SHELL` to list of known environment variables

[Alain Schlesser]

The PR wp-cli/shell-command#33 added a new environment variable WP_CLI_CUSTOM_SHELL which allows the user to override the default /bin/bash shell used.

This should be documented in https://github.com/wp-cli/handbook/blob/master/config.md#environment-variables .

[Edmund Turbin]

Created pull request: #292