Org reader: Fix block parameter reader, relax constraints

The reader produced wrong results for block containing non-letter chars
in their parameter arguments.  This patch relaxes constraints in that it
allows block header arguments to contain any non-space character (except
for ']' for inline blocks).

Thanks to Xiao Hanyu for noticing this.
Merge pull request #1290 from tarleb/fix-header-params

Org reader: Fix block parameter reader, relax constraints