[Documentation] sourceMap option produces string, not SourceMapGenerator

[jvilk]

The wiki page describing source map support states:

The outputted source map (regardless of the the sourceMapWithCode option) is a SourceMapGenerator from the Mozilla source-map library.

However, the output is actually a string containing the source map.

Should we update the documentation to reflect this, or change Escodegen's behavior to produce a SourceMapGenerator? The string behavior is probably beneficial, as it doesn't implicitly rely on a specific version of source-map.

[yanghuabei]

I also met the problem. When I use acron as parser, escodegen produces SouceMapGenerator, esprima string.