Add options to UDP proxy for helping in making doctests and dig diffs

I use them with the following script
```py
# ~/digdiff.py
import commands
import difflib
import sys

args = sys.argv[1:] or ['+short ietf.org -t A']
dns_req = ' '.join(args)

r1 = commands.getoutput('dig @0.0.0.0 -p 8053 ' + dns_req)
r2 = commands.getoutput('dig @0.0.0.0 -p 8054 ' + dns_req)

for line in difflib.unified_diff(r1.split('\n'), r2.split('\n')):
    print line
```
Run it
```sh
# start first proxy
$ python server/udp_proxy.py --dns=ns0.ietf.org --port=8053

# start second proxy
$ python server/udp_proxy.py --dns=ns0.ietf.org --port=8054 --dnslib=1

# check dig diff
$ python ~/digdiff.py +short ietf.org -t A
```

For stripping authority section and additional section from reply, run proxy

```sh
pie; py .ignored/dnslib/dnslib/server/udp_proxy.py --dns=ns0.ietf.org --answer=1
```

And then check stdout:

```
------ Request (('127.0.0.1', 61539)): ('ietf', 'org') (A)
f4c8010000010000000000000469657466036f72670000010001
  <DNS Header: id=0xf4c8 type=QUERY opcode=QUERY flags=RD rcode=None q=1 a=0 ns=0 ar=0>
  <DNS Question: 'ietf.org' qtype=A qclass=IN>
------ Reply (('64.170.98.2', 53)): ('ietf', 'org') (A)
f4c8850000010001000600020469657466036f72670000010001c00c000100010000070800040c163a1ec00c0002000100000708001b036e7331046d6961310b6166696c6961732d6e737404696e666f00c00c0002000100000708000b036e73310473656131c03fc00c0002000100000708000b036e73310479797a31c03fc00c00020001000007080006036e7330c00cc00c0002000100000708000b036e733104616d7331c03fc00c0002000100000708000b036e733104686b6731c03fc08b0001000100000708000440aa6202c08b001c000100000708001020011890126c00000000000000010002
  <DNS Header: id=0xf4c8 type=RESPONSE opcode=QUERY flags=AA,RD rcode=None q=1 a=1 ns=6 ar=2>
  <DNS Question: 'ietf.org' qtype=A qclass=IN>
  <DNS RR: 'ietf.org' rtype=A rclass=IN ttl=1800 rdata='31322e32322e35382e3330'>
  <DNS RR: 'ietf.org' rtype=NS rclass=IN ttl=1800 rdata='ns1.mia1.afilias-nst.info'>
  <DNS RR: 'ietf.org' rtype=NS rclass=IN ttl=1800 rdata='ns1.sea1.afilias-nst.info'>
  <DNS RR: 'ietf.org' rtype=NS rclass=IN ttl=1800 rdata='ns1.yyz1.afilias-nst.info'>
  <DNS RR: 'ietf.org' rtype=NS rclass=IN ttl=1800 rdata='ns0.ietf.org'>
  <DNS RR: 'ietf.org' rtype=NS rclass=IN ttl=1800 rdata='ns1.ams1.afilias-nst.info'>
  <DNS RR: 'ietf.org' rtype=NS rclass=IN ttl=1800 rdata='ns1.hkg1.afilias-nst.info'>
  <DNS RR: 'ns0.ietf.org' rtype=A rclass=IN ttl=1800 rdata='36342e3137302e39382e32'>
  <DNS RR: 'ns0.ietf.org' rtype=AAAA rclass=IN ttl=1800 rdata='2001:1890:126c:0000:0000:0000:0001:0002'>

------ Reply without additional sections:
f4c8850000010001000000000469657466036f72670000010001c00c000100010000070800040c163a1e
```

So now you can use the last line if you need a shorter doctest.