Make the cwd of the command configurable

Currently, the cwd used to run the command (effectively running the command from this location) is hardcoded as the playbook directory (or process.cwd() if the playbook directory is not set for some reason.

It should be possible to specify a different cwd using the build.cwd key in the Assembler configuration. This value should be expanded like any other configurable path in Antora.

Example:

build:
  cwd: ~+

(Always uses the current directory at the time Antora was launched).

Be sure to update the documentation accordingly.