Various improvements to the /wattsi endpoint

[domenic]

In the course of working on adding a full /html-build endpoint, a few improvements came up which we can apply ahead of time to just the /wattsi endpoint:

• Improve README documentation.
• Abstract out the code for turning query parameters into boolean arguments.
• Use more modern JS features.
• Put temporary directories inside the OS temp directory.
• Add a new helper for spawning a command-line program while capturing its output. This fixes output.txt should contain interleaved stdout/stderr instead of stdout + \n\n + stderr #3.
• Perform cleanup file deletions in parallel and log errors if they occur while deleting.
• Move from deprecated --production npm install argument to modern --omit=dev.