feat: allow discovery of mDNS services

[caarlos0]

this builds on #48, adding the discovered nodes as "seed" entrypoints to sshparse, which will in turn apply Host rules as expected.

closes #48
refs #135
cc/ @wizardishungry

---

to try it out:

create with /etc/avahi/services/ssh.service:

<?xml version="1.0" standalone="no"?>
<!DOCTYPE service-group SYSTEM "avahi-service.dtd">
<service-group>   
  <name replace-wildcards="yes">%h</name>
  <service>
    <type>_ssh._tcp</type>
    <port>22</port>
  </service>
</service-group>

then

service avahi-daemon restart

# to test:
avahi-browse -a

you can then run wishlist with -z and it should load all _ssh._tcp services accessible...