Add automated checking for non-escaped --

This checks to see if any man output has [em] in it, as that will not
be correctly show in ASCII. Unfortunately the check will depend on
whether the other man check is done, but I didn't want to introduce
yet another short script for this (can be done, no issue).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>