Rename block_math to block_stem

According to https://github.com/asciidoctor/asciidoctor/commit/e6b8e14ab69d0bb4bd00001b144878e88d872103.
Fix html5/document - don't load MathJax manually

Reflects https://github.com/asciidoctor/asciidoctor/commit/befc02bc5a9a30625b4a118d418e99f9aac68ac1.
Fix html5/block_stem - set default subs on stem block

Reflects https://github.com/asciidoctor/asciidoctor/commit/f44e70af9e9a41f22264930d1514a2c5451bc3be
Add examples/html5/block_stem
Merge pull request #96 from jirutka/patch-10

Fix html5 math/stem