Extract isGetString: add option moduleMethodStringArgumentIndex

[Fridus]

Real usage :

Extract for the method gettextCatalog.getStringForLanguage. The text is the second argument

gettextCatalog.getStringForLanguage(
          lang,
          'text'
        )

const gettextExtractor = new Extractor({
  moduleMethodString: "getStringForLanguage",
  moduleMethodStringArgumentIndex: 1
})