Avoid infinite loop and patch exponential backoff edge case

[gittyeric]

This changes a few snags I hit as a noob to the API that I figured I'd toss up as a PR:

• Make exponential backoff not degrade to a sum when minTimeout is <= 0
• Auto-switch to forever mode when retries is Infinity. This is still problematic for the timeouts method but at least it patches the primary use-case.

I hope I got the version change right, let me know if you need anything else, thanks!

[codecov]

Codecov Report

Merging #68 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master     #68   +/-   ##
======================================
  Coverage    90.2%   90.2%           
======================================
  Files           2       2           
  Lines         143     143           
======================================
  Hits          129     129           
  Misses         14      14

Impacted Files	Coverage Δ
lib/retry.js	98% <100%> (ø)	⬆️

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[tim-kos]

Great, thank you!