Added ability to pass custom resolver function through to react-docgen

[mrsharpoblunto]

Added ability for markdown renderer to handle more than one component per file as some resolver functions may return more than one component per file.

This fixes https://github.com/AdRoll/gulp-react-docs/issues/1, and adds some useful flexibility while preserving the existing behaviour and markdown appearance when using the default resolver function.

[mikaoelitiana]

Hello, Can you please merge this? This is a really required feature.

Thanks