Add subscribe method to action to abbreviate addons

[nhunzaker]

This PR adds a new subscribe method for quickly adding a bunch of action callbacks. Long term, this moves actions closer to the Observable interface, but it also cleans up the action form and button pretty nicely.

I also added some tests to gain some missing coverage.

[codecov-io]

Codecov Report

Merging #408 into scheduled-model will decrease coverage by 1.23%. The diff coverage is 100%.

@@                 Coverage Diff                 @@
##           scheduled-model     #408      +/-   ##
===================================================
- Coverage            87.32%   86.08%   -1.24%     
===================================================
  Files                   45       45              
  Lines                 1270     1301      +31     
  Branches               232      244      +12     
===================================================
+ Hits                  1109     1120      +11     
- Misses                 135      147      +12     
- Partials                26       34       +8

Impacted Files	Coverage Δ
packages/microcosm-preact/src/action-button.js	100% <100%> (ø)	:arrow_up:
packages/microcosm/src/action.js	100% <100%> (ø)	:arrow_up:
packages/microcosm/src/addons/action-form.js	78.04% <100%> (-21.96%)	:arrow_down:
packages/microcosm/src/addons/action-button.js	77.77% <100%> (-22.23%)	:arrow_down:
packages/microcosm-preact/src/action-form.js	100% <100%> (ø)	:arrow_up:
packages/microcosm/src/addons/indexing.js	92.68% <0%> (-7.32%)	:arrow_down:

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