Unable to use ES3 target option - propose using strings for setting the target option instead.

[sccolbert]

This line will fail if the target option is set to ts.ScriptTarget.ES3, because that value is 0 and will test false:
https://github.com/SitePen/dts-generator/blob/master/index.ts#L104

Futhermore, it's not completely safe for a user to require typescript and pass along that enum value, since it may come from a version of typescript which is different than the one imported by dts-generator. The safest/simplest way to fix both issues is to use a string for the target option.