#92 Warn of problematic characters.

[Günter Milde]

Unprintable characters like NULL in the rST source are most probable an indication of a problem
(corrupt source file, wrong encoding, ...).
The same goes for combining characters at the start of a line, etc.
It may be helpful, if Docutils issued a Warning for problematic characters in the source (except for literals).

[Günter Milde]

Idea moved to the developer documentation (todo.rst).