Parallelize actions

[bmuenzenmeyer]

I've written up a file which calls praxis-scripts numerous times for each command, and I'm realizing that as time goes on the number of commands will only grow. As they occur in precommit, it's going to take longer and longer to commit as our documentation grows.

We can cut this down substantially by running the actions at the same time. Changes should be created in batches in memory, and then replacements can occur in series after all actions have completed.

Some actions can not / should not be performed at the same time, such as git. These actions can accept a parameter ("parallel": false?) to make them run in a separate series of work.

A huge thanks to our most recent contributor:
<!--
  CODEBLOCK_START
  {
    "type": "command",
    "value": "git log -1 --pretty=format:'%an'",
    "parallel": false
  }
-->

<-- CODEBLOCK_END -->