Add basic checking of mx records for mailto links

[tomtom5152]

Helps ensure that the domain is registered and at least partially configured to recieve email.

[codecov-io]

Codecov Report

Merging #57 into master will decrease coverage by 2.21%. The diff coverage is 60.97%.

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
- Coverage   95.67%   93.45%   -2.22%     
==========================================
  Files          17       17              
  Lines         902      917      +15     
==========================================
- Hits          863      857       -6     
- Misses         34       49      +15     
- Partials        5       11       +6

Impacted Files	Coverage Δ
htmltest/check-link.go	91.6% <60.97%> (-3.5%)	:arrow_down:
htmltest/htmltest.go	89.18% <0%> (-8.63%)	:arrow_down:
htmltest/options.go	89.06% <0%> (-0.17%)	:arrow_down:
htmldoc/document.go	88.37% <0%> (+0.27%)	:arrow_up:

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[tomtom5152]

Coverage has decreased as I can't think of a way to trigger the errors to execute those lines of code. In theory they should never execute, but it is a bit of catch all error handling in a default and else statement.

[davidtaylorhq]

Does this pass for the case that a domain has no MX records, but has a valid A/CNAME record? (I believe it should)

[tomtom5152]

I believe you are correct according to this RFC 5321 Section 5 but that doesn't current exist.

[wjdp]

Looking at the changes coverage is fine.

Re. the RFC my understanding is as follows:

Checking items off as I read them

• [x] DNS lookup must be performed
• [x] First check for MX
• [x] If CNAME present restart from beginning
• [x] If neither, fallback to A or AAAA
• [x] If NXDOMAIN then error
• [ ] If haven't found a valid record then error

[tomtom5152]

Rebased and made the changes requested.