[node.js] Add inSourceMapFromString option so one can pass in the sourcemap contents in inSourceMap.

I have both a js file and sourcemap for it in strings in my app. In order to call minify() I either need to write the source map to a temp file (yuck), or switch to the complicated API (feels overwhelming).

This small patch lets me pass in the sourcemap by adding the new option `inSourceMapFromString`, which is analogous to the `fromString` option.