Configurable refuse when SPF is none

[jcbf]

Implements #41

[coveralls]

Coverage decreased (-0.6%) to 86.437% when pulling 230d4e3 on jcbf/refuse-spf-none into 9699193 on master.

[codecov]

Codecov Report

Merging #42 into master will decrease coverage by 0.57%.
The diff coverage is 55.55%.

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
- Coverage   86.84%   86.27%   -0.58%     
==========================================
  Files           1        1              
  Lines         479      488       +9     
  Branches      167        0     -167     
==========================================
+ Hits          416      421       +5     
- Misses         63       67       +4

Impacted Files	Coverage Δ
smf-spf.c	86.27% <55.55%> (-0.58%)	⬇️

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