Force value to be a string so PostCSS does not append px

[lukebennett88]

This PR addresses this issue: #2

If someone chooses to configure the values for lineClamp but uses a number instead of a string, PostCSS will convert the unitless value to a pixel value:

e.g. tailwind.config.js:

module.exports = {
  theme: {
    lineClamp: {
      1: 1,
      2: 2,
      3: 3,
    },
  },
};

Outputs:

.line-clamp-1 {
  overflow: hidden;
  display: -webkit-box;
  -webkit-box-orient: vertical;
  -webkit-line-clamp: 1px; /* This should be 1 */
}

.line-clamp-2 {
  overflow: hidden;
  display: -webkit-box;
  -webkit-box-orient: vertical;
  -webkit-line-clamp: 2px; /* This should be 2 */
}

.line-clamp-3 {
  overflow: hidden;
  display: -webkit-box;
  -webkit-box-orient: vertical;
  -webkit-line-clamp: 3px; /* This should be 3 */
}

[RobinMalfait]

Hey! Thank you for your PR!
Much appreciated! 🙏

Since the line-clamp doesn't accept px values (unlike line-height where you can have px and non-px values), I think this is a nice improvement. Makes the config file a bit cleaner!

[ghost]

What about the example in readme? Let it stay as is or a shorter version?