[0.5] Move container component to a built-in plugin

[adamwathan]

Resolves #256.

Prior to this PR, it was impossible to disable the .container component because it's sort of a special snowflake. It's not a utility class and doesn't support any of the variants that the rest of our utilities support; it's actually a component, and has different styles at different breakpoints.

With the introduction of the new plugin system and the @tailwind components directive, it makes sense to inject the container styles into the components section of the user's stylesheet, and making the container a built-in plugin instead of a special-case hard-coded utility means it's now trivial to disable; you simply remove it from the plugin list:

  plugins: [
-   require('tailwindcss/plugins/container')(),
  ],

This is a breaking change so slated for 0.5.