standalone: strip ansi codes in logs

This fixes a bug when the program output contain some ansi code, like
the \e[0;31m ansi code to display text in color on the terminal. These
characters are not allowed in XML so if they end up in the XML-RPC
messages this cause the underlying expat library to complain that the
stream is not well-formed (invalid token).

Fortunately, supervisor supports this and has a `strip_ansi` option that
we can set in the config to prevent these problems.