Incorrectly resolves absolute source mapping url as relative to file

The `supportRelativeURL` function always assumes mappings are relative, even when they may be an absolute URI.

Given:
- Build 0.3.2 of source-map-support
- a **file** of `c:\dir\a.min.js`
- a **url** of `file:///C:/dir/a.js`

The result is `C:\dir\file:\C:\dir\a.js` when it should be `C:\dir\a.js`.

The issue stems from the fact node's `path.resolve` function does not interpret file URIs.