support for custom constants and values

[Mte90]

As #14 in this way we can have custom constants with also custom values!

boilerplate:
        repo: git@github.com:Varying-Vagrant-Vagrants/custom-site-template.git
        custom:
			constants:
				WP_DEBUG: true
				WP_DEBUG_LOG: true
				WP_DISABLE_FATAL_ERROR_HANDLER: true # To disable in WP 5.2 the FER mode
        nginx_upstream: php72
        hosts:
            - boilerplate.test

[Mte90]

I cannot merge, say merging is blocked