Support changing settings via command line/environment

[SpecLad]

Currently, the only way to change pushpin settings is to edit the config (or use a custom config file). This isn't very convenient, especially when using the Docker image. You can, in principle, do something like this:

docker run fastly/pushpin bash -c '
    sed -i 's/^my_option=.*/my_option=my_value/' /etc/pushpin/pushpin.conf
    exec pushpin --merge-output
'

but it's not exactly ergonomic.

It would be great if you could override specific config settings via the command line, something like:

pushpin -c runner.allow_compression=true

Alternatively, environment variables would work too.

PUSHPIN_RUNNER_ALLOW_COMPRESSION=true pushpin