Webpacker: Add tailwind.config.js

[dixpac]

During webpack installation generate tailwind.config.js.

This commit places that file inside stylesheets/ dir, and link to it
inside postcss.config.js.

This is the blank Tailwind confg file that one can extend
to bring its own esthetics.

[ghiculescu]

What do you think about enabling purging in this file? That way the webpacker behavior would match the asset pipeline behavior out of the box.

I think the change would just be this:

-  purge: [],
+  purge: {
+    enabled:  process.env.NODE_ENV === 'production',
+    content: ['./app/views/**/*.*', './app/helpers/**/*.*'],
+  },

But wouldn't mind someone sanity checking that.

[dixpac]

@ghiculescu something like this #40 ?

[ghiculescu]

@dixpac yes, that's great!