Add support for forcing a specific ip protocal version

[ArenM]

Support specifying the ip protocol version to use, like to curl -4 and curl -6.
fixes #252

[codecov]

Codecov Report

Merging #253 (ab35df5) into master (a9d71aa) will decrease coverage by 0.45%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #253      +/-   ##
==========================================
- Coverage   72.53%   72.07%   -0.46%     
==========================================
  Files          49       49              
  Lines        2443     2435       -8     
==========================================
- Hits         1772     1755      -17     
- Misses        671      680       +9     

Impacted Files	Coverage Δ
src/client.rs	70.49% <ø> (-0.28%)	⬇️
src/config/mod.rs	56.66% <0.00%> (-4.05%)	⬇️
src/request.rs	27.45% <0.00%> (-0.55%)	⬇️
src/task.rs	90.90% <0.00%> (-1.69%)	⬇️
src/body.rs	75.38% <0.00%> (-0.81%)	⬇️
src/handler.rs	71.31% <0.00%> (-0.06%)	⬇️
src/text.rs	68.08% <0.00%> (ø)
src/agent.rs	68.58% <0.00%> (ø)
src/response.rs	82.35% <0.00%> (ø)
... and 1 more

---

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 a9d71aa...ab35df5. Read the comment docs.

[sagebind]

Thanks for contributing! Just a few suggestions on docs and naming, but otherwise looks good!

[ArenM]

Thanks for the comments, I'm not good at writing docs or naming things.

[sagebind]

Looks good, thanks!