modules/find.py: Disallow ".", ":" and "-" as separators

Rationale: they may occur in IP addresses or hostnames, and I just
recently accidentally triggered this:

    <lyoko1> what's up with cadoth
    -snip-
    <lyoko1> s.cadoth.net:2200
    <jenn1> lyoko1 probably meant to say: what's up with net:2200