feat(cmd) add embed option

[princjef]

Adds a new --embed option which allows for documentation to be embedded in an existing markdown file

Fixes #55

[codecov-commenter]

Codecov Report

Merging #59 (9d5efb8) into master (08e0e91) will increase coverage by 0.55%.
The diff coverage is 54.54%.

@@            Coverage Diff             @@
##           master      #59      +/-   ##
==========================================
+ Coverage   54.02%   54.57%   +0.55%     
==========================================
  Files          16       17       +1     
  Lines        1516     1552      +36     
==========================================
+ Hits          819      847      +28     
- Misses        633      639       +6     
- Partials       64       66       +2     

Impacted Files	Coverage Δ
cmd/gomarkdoc/output.go	50.49% <50.49%> (ø)
cmd/gomarkdoc/command.go	77.27% <100.00%> (+5.39%)	⬆️

---

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 08e0e91...9d5efb8. Read the comment docs.