Exporting sourcemaps

Fixes #30
Also why is it that some times it is needed to pre-fill it and sometimes not (e.g https://github.com/apiaryio/snowcrash/pull/211/files#diff-c0741cebd5e7a6877ebd311f5c5e62c8R106) ? 

I would like to unify the initialization of in-place parserresults instances