Support custom importers

[Slava]

Add hooks for an option called importer - an object that implements find and readFile operations in a custom way to enable new import syntax defined by the plugin.

This is useful for better cross-package imports in frameworks and package systems, such as Meteor.

I am open to making improvements to the PR, adding docs, etc, if somebody gives me a sign off as "this is a good feature we want to merge to Stylus" by one of the maintainers.

[Panya]

It's good intention. But I think implementation could be improved. E.g. we could extract current logic of importFile to the default importer (lib/importer.js). Also I would like to see tests/docs.

[Slava]

Excuse me, I didn't find a file called /lib/importer.js, would you please clarify?

[Panya]

I mean it would be nice to extract this logic to a file that should be placed at lib/importer.js.

[Slava]

Gotcha, I will work on this later this week with tests and docs. Thanks, comrade.