Added an extension `short_subsuperscripts` which modifies the behavior
of `subscript` and `superscript`, allowing subscripts or superscripts containing only
alphanumerics to end with a space character (eg. `x^2 = 4` or `H~2 is
combustible`). This improves support for multimarkdown. Closes#5512.
Add `Ext_short_subsuperscripts` constructor to `Extension` [API change].
This is enabled by default for `markdown_mmd`.
e37cf4484d