This fixes the org-reader's handling of sub- and superscript
expressions. Simple expressions (like `2^+10`), expressions in
parentheses (`a_(n+1)`) and nested sexp (like `a_(nested()parens)`) are
now read correctly.
Support all of the following variants as valid ways to define inline or
display math inlines:
- `\[..\]` (display)
- `$$..$$` (display)
- `\(..\)` (inline)
- `$..$` (inline)
This closes#1223. Again.
Text such as /*this*/ was not correctly parsed as a strong, emphasised
word. This was due to the end-of-word recognition being to strict as it
did not accept markup chars as part of a word. The fix involves an
additional parser state field, listing the markup chars which might be
parsed as part of a word.
