lib/options: teach showOptions about funny option names

Handle the case where options have funny symbols inside of them.

Example:

If I reference the following attribute without it being defined:

  security.acme.certs."example.com".webroot

I now get the error:

    The option `security.acme.certs."example.com".webroot' is used but
    not defined.

where before I got:

    The option `security.acme.certs.example.com.webroot' is used but
    not defined.

which is not true.