Implement eager postcss preprocessing

[arggh]

Uses PostCSS preprocessing, if postcss is configured & available and there is no conflicting lang attribute present in the <style> tag.

Fixes #5

[zodern]

For version 2 of melte-compiler, we should remove the checks for attribute.lang and always run postcss to match what svelte-preprocessor does (it runs the preprocessor for the lang if there is one, and then runs postcss).

To avoid introducing a backwards breaking change now, could we add an option for this behaviour?

[arggh]

Sure, that's what I suggested in #5

What would you like the config option to be named? I'm happy to update the PR.

[zodern]

I'm not sure what a good name would be. Maybe something like alwaysUsePostCSS?

[zodern]

Meteor 2.6.1 adds built-in support for postcss in its standard-minifier-css package, and when svelte's css option is disabled, all of the styles from svelte will be passed through standard-minifier-css.

I'm thinking of deprecating melte's postcss integration, and instead recommending disabling the css option if you want to use PostCSS. What do you think? Are there still benefits to having melte run postcss plugins?