call `tailwindcss:config` hook after all modules are initialised

[Daniel Roe]

At the moment, tailwindcss:config is called synchronously when this module runs, meaning other modules which hope to use tailwindcss or modify its configuration must be registered before it.

However, dependencies between modules aren't ideal. Instead, we should register a hook in modules:done that finally calls tailwindcss:config. Alternatively, we could also call tailwindcss:config directly in getContents of a template.

[Inesh Bose]

I'm looking to address this in #818 as that is a complete rewrite of loading the config. 👍