[read_dta] for strl import use std::vector<std::string> to push back. closes #87

[JanMarvin]

and not Rcpp::CharacterVector

Included test by @hans-elliott99

[JanMarvin]

All strls should be Unicode, but converting a unicode std-string-vector to latin1 R might cause broken strings. Not sure that we can always rely on iconv to create valid latin1 strings in such case. Non unicode R is slowly dying out, but there will be some old Windows installations for quite some time. But I recommend to cross this bridge once we reach it.