Allow escalus_tcp to filter stanzas
Add 'not'/1 higher-order function to escalus_pred
Sanitize Escalus config global variable names

- `escalus_server` - the XMPP domain used by the system
- `escalus_host` - the machine to connect to

In case of unavailable `escalus_host` the host to connect to is
`escalus_server`. It was the other way around - quite counter-intuitive.
Add todos about the configuration
Use the domain/server to setup the stream

The host should only be used for connecting the TCP socket.
As soon as XMPP is involved the XMPP domain (also called server) is
important.
Allow the whole range of inet addresses as host
Fix whitespace
Expect Config to be passed to is_mod_register_enabled

External API change - bump version number.