Allow configuring custom DNS TTL

[ThinkChaos]

I changed the tests to use a random but common TTL on each run. Maybe you'd prefer a separate test for the TTL explicitly.
Also, I haven't updated the docs.

[codecov]

Codecov Report

Merging #370 (25a088f) into development (9fcb11e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           development     #370   +/-   ##
============================================
  Coverage        96.17%   96.17%           
============================================
  Files               30       30           
  Lines             2250     2251    +1     
============================================
+ Hits              2164     2165    +1     
  Misses              56       56           
  Partials            30       30           

Impacted Files	Coverage Δ
config/config.go	91.48% <ø> (ø)
resolver/custom_dns_resolver.go	100.00% <100.00%> (ø)

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[0xERR0R]

Thanks for your work! It looks good. Do you want to add a description to configuration.md and docs/config.yml? If not, it is ok, I can do it.

[ThinkChaos]

I added the docs!