[WIP] Add support for auto switch URL.

[gucong3000]

Closes #60

[codecov-io]

Codecov Report

Merging #76 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #76   +/-   ##
=======================================
  Coverage   92.42%   92.42%           
=======================================
  Files           2        2           
  Lines          66       66           
=======================================
  Hits           61       61           
  Misses          5        5

Impacted Files	Coverage Δ
index.js	90.19% <100%> (ø)	⬆️

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[coveralls]

Coverage increased (+1.5%) to 92.424% when pulling 06103bd on gucong3000:auto_switch_url into 777d7db on matheuss:master.

[rawr51919]

There shouldn't be anything breaking this API when this code is allowed, and from the looks of this code, it'll help greatly. I approve.