Expose sourceType in parser option

[Options](https://github.com/benjamn/recast/blob/master/lib/options.ts#L7) should have a property `sourceType`. Otherwise there is no way of changing the default value [here](https://github.com/benjamn/recast/blob/master/lib/parser.ts#L33).

Start with the Options definition in lib/options.ts and the default sourceType handling in lib/parser.ts. Trace how parser options are passed through, then verify that sourceType can be supplied through Options and controls the parser setting described in the issue.

Written by the indexing model from the issue text.