Migrate XMLRPC usage to xmlrpc2

[thadguidry]

Problem: RCurl uses an obsolete version of OpenSSL which causes failures posting to WP sites that now restrict anything lower than TLS 1.2

Solution: It might be advantageous to switch the implemention in RWordPress from using XMLRPC to instead using xmlrpc2 which uses curl binding package and xml2. https://cran.r-project.org/web/packages/xmlrpc2/index.html

Additional Context: #44 RCurl can only use obsolete transport security