Add support for default duration and timing function to transition property utilities

[adamwathan]

This PR makes it possible to define a default duration and timing function that is automatically applied any time you add any transitionProperty utility, using the DEFAULT keyword under the transitionTimingFunction and transitionDuration sections of your theme:

// tailwind.config.js
module.exports = {
  theme: {
    extend: {
      transitionDuration: {
        DEFAULT: '150ms',
      },
      transitionTimingFunction: {
        DEFAULT: 'cubic-bezier(0.4, 0, 0.2, 1)',
      },
    }
  }
}

This config for example lets you simplify this sort of code:

- <button class="... transition ease-in-out duration-150">
+ <button class="... transition">

You can still easily override the defaults by layering on duration or timing function utilities:

<button class="... transition ease-out duration-300">

This PR also makes 150ms the default duration by default, and the ease-in-out curve the default transition timing function curve by default.