Feature/postcss option as function

[cybercase]

Hello @ai

This PR helps to address the issue #8 . A full example is provided in README.
Let me know what you think. Thanks

Stefano

[ai]

Awesome.

Maybe we need to use function by default?

Can you send a PR with default function and special section with onImport example?

[cybercase]

Thanks for merging!
Of course I can send another PR

Are the following changes ok?

• Update of "Usage" and "Plugins Pack" sections to use function instead of Objects or Array.
• Update of "Plugin Function" section with an "Integration with postcss-import"

[ai]

Yeap :)

[ai]

@cybercase I am waiting for second PR to make a release :)