feat(renderer): allow templates to be retrieved from URL

[andrewrynhard]

Fixes #39.

[codecov]

Codecov Report

Merging #41 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #41   +/-   ##
=======================================
  Coverage   68.62%   68.62%           
=======================================
  Files           1        1           
  Lines          51       51           
=======================================
  Hits           35       35           
  Misses         13       13           
  Partials        3        3

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