CRAN release version 1.0.18

What's Changed

• Replace Rf_error() call with Rcpp::stop() call by @Kss2k in #129
• Add categorical argument to modsem_mplus() by @Kss2k in #130
• Add some support for the <~ operator by @Kss2k in #131
• Use standardized vcov in plot and simple slopes functions by @Kss2k in #132
• Format digits in simple slopes by @Kss2k in #133
• Refactor code for compositeRVCOV() by @Kss2k in #134

Full Changelog: v1.0.17...v1.0.18