Allow context variable references to be used in run command

To make the run command more dynamic, and to avoid the need to set environment variables in some cases, the context variable reference syntax that was introduced in #24 (closed) for setting environment variables should also be available in the command string.

As part of this change, document that environment variable references (e.g., $USER) can be used in the command string.

We should also reconsider the behavior of rewriting the node base call. Instead, the NODE environment variable should be set (if not already set) so it can be used in the command string (e.g., $NODE generate-files.js). Using this environment variable ensures that Collector uses the same Node.js executable that was used to launch Antora.