Ability to override configuration (e.g. from CLI)

[denisdefreyne]

See https://gitlab.com/gitlab-org/gitlab-docs/-/issues/81#note_540515256.

Being able to override configuration bits could make sense for data sources, where different machines might have their data stored in different locations.

Some ideas for how to do that:

• Interpret ERB in the configuration file. This would not be a backwards-compatible change.

• Introduce a syntax for specifying values coming from environment variables, e.g. content_dir: @@ENV['NANOC_CONTENT_DIR']. Feels hacky.

• Allow specifying config overrides via the CLI, e.g. nanoc co --config-override=data_sources[0].content_dir=../somewhereelse. With data sources being an array, this is a little brittle, having to specify [0] rather than something more descriptive.

(Add your own ideas in the comments.)