Org reader: Fix parsing of sub-/superscript expressions

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.
Merge pull request #1231 from tarleb/org-fix-subexpr

Org reader: Fix parsing of sub-/superscript expressions