Prepare for 3.1.0 release
Prepare for 3.1.0 release

jeremykenedy/html-to-markdown,deadflowers/html-to-markdown,nickcernis/html-to-markdown,thephpleague/html-to-markdown,thephpleague/html-to-markdown,jarnix/html-to-markdown,TomzxForks/html-to-markdown,nickcernis/html-to-markdown