Experimental: Support API Blueprint output

[tcnksm]

This PR adds experimental support of API blueprint format output.

From .apib and https://github.com/subosito/snowboard, you can generate the following html documentation,

[codecov-io]

Codecov Report

Merging #8 into master will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##           master       #8   +/-   ##
=======================================
  Coverage   99.04%   99.04%           
=======================================
  Files           3        3           
  Lines         209      209           
=======================================
  Hits          207      207           
  Misses          2        2

Impacted Files	Coverage Δ
httpdoc.go	100% <ø> (ø)	:arrow_up:
template.go	100% <100%> (ø)	:arrow_up:

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