Add !dns command to query the DNS

[Roobarb]

!dns nos.nl
should return "145.58.28.175, 145.58.29.114" (A records only by default)

and other types of records can be specified:
!dns AAAA nos.nl
should return "2a02:458:101:28:10:28::af"
!dns MX nos.nl
should return "12 mx-a.mf.surf.net, 12 mx-1.mf.surf.net."

[casdr]

If you don't want to do exec stuff locally, http://digweb.herokuapp.com/ might be useful:

$ curl http://digweb.herokuapp.com/ -d "nos.nl"

; <<>> DiG 9.9.5-3ubuntu0.9-Ubuntu <<>> nos.nl
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 51881
;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;nos.nl.                IN  A

;; ANSWER SECTION:
nos.nl.         3600    IN  A   145.58.28.175
nos.nl.         3600    IN  A   145.58.29.114

;; Query time: 111 msec
;; SERVER: 172.16.0.23#53(172.16.0.23)
;; WHEN: Sun Nov 06 21:58:24 UTC 2016
;; MSG SIZE  rcvd: 67

$ curl http://digweb.herokuapp.com/ -d "nos.nl MX"

; <<>> DiG 9.9.5-3ubuntu0.9-Ubuntu <<>> nos.nl MX
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 42452
;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;nos.nl.                IN  MX

;; ANSWER SECTION:
nos.nl.         300 IN  MX  12 mx-a.mf.surf.net.
nos.nl.         300 IN  MX  12 mx-1.mf.surf.net.

;; Query time: 108 msec
;; SERVER: 172.16.0.23#53(172.16.0.23)
;; WHEN: Sun Nov 06 21:59:18 UTC 2016
;; MSG SIZE  rcvd: 88