Add Connection Cache TTL

[gsquire]

This patch uses the newly exposed curl configuration allowing for a client to set up a connection cache TTL.

Closes #93

[gsquire]

Sorry, I think rustfmt made a few changes to the style. I can revert those chunks if you would like.

[codecov]

Codecov Report

Merging #237 into master will decrease coverage by 0.24%.
The diff coverage is 22.22%.

@@            Coverage Diff             @@
##           master     #237      +/-   ##
==========================================
- Coverage   71.09%   70.85%   -0.25%     
==========================================
  Files          44       44              
  Lines        2128     2134       +6     
==========================================
- Hits         1513     1512       -1     
- Misses        615      622       +7     

Impacted Files	Coverage Δ
src/config/mod.rs	61.81% <0.00%> (-1.15%)	⬇️
src/client.rs	64.67% <28.57%> (-1.68%)	⬇️

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[sagebind]

Looks great, thanks for working on this!

Don't worry about the formatting too much, it isn't a problem. One of these days I'll add a rustfmt config file if I cared about being consistent...