Avoid hitting the file system

[SimenB]

I'd like to use this module in Jest. But this module tries to look at the filesystem and read sourcemaps on its own, which breaks down. I'd love an alternative API that allows me to pass in:

• raw source code (from "disk")
• the source code that was executed/instrumented
• the source map for this

I cobbled this together quickly, which allowed me to get a PoC working in Jest: https://github.com/SimenB/v8-to-istanbul/commit/6d1560b890f64b05e476e64719e3ffd26ddf357c