Refine microcosm-http. and microcosm-dom Add file-uploader example

[nhunzaker]

This commit moves the file-upload example from microcosm-http over to the examples directory and updates it with a cleaner interface.

In the process of doing this, I made a few improvements that I've pointed out within the file diff.

---

[efatsi]

Cool stuff, like the way that click and submit have simplified down.

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@afbf1cb). Click here to learn what that means. The diff coverage is 100%.

@@           Coverage Diff            @@
##             master    #509   +/-   ##
========================================
  Coverage          ?   99.7%           
========================================
  Files             ?      27           
  Lines             ?     672           
  Branches          ?     133           
========================================
  Hits              ?     670           
  Misses            ?       2           
  Partials          ?       0

Impacted Files	Coverage Δ
packages/microcosm-dom/src/action-button.js	100% <100%> (ø)
packages/microcosm/src/history.js	100% <100%> (ø)
packages/microcosm-dom/src/presenter.js	100% <100%> (ø)
packages/microcosm-dom/src/action-form.js	100% <100%> (ø)
packages/microcosm/src/subject.js	100% <100%> (ø)
packages/microcosm-http/src/http.js	100% <100%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update afbf1cb...d72a0ef. Read the comment docs.

[nhunzaker]

Thanks, guys!