Support data-uri format

[source-map spec](https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit) says:

```
Note: <url> maybe a data URI.  Using a data URI along with “sourcesContent”
allow for a completely self-contained source-map.
```

And this pull-req implements data-uri format. Can you review it please?