add ps:wait command and supporting function

[jdowning]

This adds a function ps_wait that is mostly pulled from uhoh-itsmaciek/heroku-ps-wait. This allows a user to run heroku ps:wait to wait for all dynos to be up and running. Additionally, this adds a flag to ps:restart (--wait) to run the ps_wait function after initializing the restart.

ᐅ h ps:wait                
Waiting for every dyno to be running v977... 37 / 37, done

ᐅ h ps:restart web && h ps:wait --type web 
Restarting web dynos on ⬢ my-heroku-app... done
Waiting for every web dyno to be running v977... 2 / 2, done

ᐅ h ps:restart --wait
Restarting dynos on ⬢ my-heroku-app... done
Waiting for every dyno to be running v977... 37 / 37, done

[jdx]

looks great! but we should really have a test for at least the happy path of ps:wait. I can give you a hand writing that if you like.

[codecov[bot]]

Codecov Report

Merging #208 into master will decrease coverage by 2.05%. The diff coverage is 20.93%.

@@            Coverage Diff             @@
##           master     #208      +/-   ##
==========================================
- Coverage   95.72%   93.67%   -2.06%     
==========================================
  Files          76       78       +2     
  Lines        1521     1564      +43     
  Branches      289      301      +12     
==========================================
+ Hits         1456     1465       +9     
- Misses         62       85      +23     
- Partials        3       14      +11

Impacted Files	Coverage Δ
src/ps_wait.js	6.06% <6.06%> (ø)
src/commands/ps/restart.js	93.75% <66.66%> (-6.25%)	:arrow_down:
src/commands/ps/wait.js	71.42% <71.42%> (ø)

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