Add "etag" response header

[TooTallNate]

Utilize the etag module on npm to send an "etag" response header.

[leo]

We want to generate strong ETags, as the content is 100% static. In turn, I'd suggest generating a sha1 from the stream.

Also, from the package docs (but it's more or less obvious):

By default, a strong ETag is generated except for fs.Stats, which will generate a weak ETag.

[codecov]

Codecov Report

Merging #94 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #94   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           2      2           
  Lines         324    324           
=====================================
  Hits          324    324

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[codecov]

Codecov Report

Merging #94 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #94   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           2      2           
  Lines         324    343   +19     
=====================================
+ Hits          324    343   +19

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

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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