Fix container plugin screen order and duplication

[stidges]

This PR fixes an issue where using the .container class with custom screen breakpoints would result in the container always having a max-width of the last defined screen. Additionally, when multiple screens would have the same min-width, the container definition would be duplicated.

I ran into this when using the following tailwind.config.js:

module.exports = {
  important: true,
  theme: {
    extend: {
      screens: {
        'sm-only': { min: '640px', max: '767px' },
      },
    },
  },
  variants: {},
  plugins: [],
};

In this case, the .container class would always have max-width: 640px defined, because it is the last defined breakpoint for the class. It would also result in duplication of the @media (min-width: 640px) breakpoint definition.

[adamwathan]

Thanks @stidges!