Fix broken SourceNode link
More flexible source map passing

- Make it “official” that passing a source map as a string works (it
  just happened to work before).
- Allow passing a source map as anything with a `.toJSON()` method (such
  as a `SourceMapGenerator`).
- Rename `file.content` to `file.code`

To be consistent with `SourceNode.toStringWithSourceMap()` and rework
(`css.stringify`). After all, in reality the content is going to be
code, so we might just as well call it that. "code" is also shorter than
"content".
Release v0.2.0