Add options for checks for domain resolvability.

[skalee]

An URL is considered resolvable when its host name is an IP address (IPv6 is supported), or is a domain for which there is a DNS record (currently, all record types are accepted, be it A, CNAME, or MX, or TXT).

Also see #12.

[codecov[bot]]

Codecov Report

Merging #81 into master will increase coverage by 0.2%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master      #81     +/-   ##
=========================================
+ Coverage   97.68%   97.89%   +0.2%     
=========================================
  Files          14       14             
  Lines         303      333     +30     
=========================================
+ Hits          296      326     +30     
  Misses          7        7

Impacted Files	Coverage Δ
lib/uri_format_validator/reacher.rb	96.15% <100%> (+2.03%)	:arrow_up:
lib/uri_format_validator/constraints.rb	97.29% <100%> (+0.15%)	:arrow_up:
spec/reacher_spec.rb	100% <100%> (ø)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 9508df7...96d941b. Read the comment docs.

[skalee]

Ouch, there is some problem with URI regexp. This one will be merged-in after doing #75.

[skalee]

Currently, this one is full of work-in-progress things which I pushed to migrate things from one computer to another.

[skalee]

Let's hold on with this one for this reason: https://github.com/riboseinc/uri_format_validator/issues/12#issuecomment-339617423