prevent catastrophic backtracing when doing regexes

This changeset no longer allows full regexes in `s/term/replacement/` in the `meant_to_say` plugin as there is no way to prevent catastrophic backtraces in regular expressions in Python.

All tests passed without alteration, effectively making this change (somewhat) backwards compatible with the
previous implementation.

This triggers the backtracing with the current implementation (that this PR fixes):

    alfredodeza aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    alfredodeza s/(a+a+)+b/this will asplode/