Support default values for placeholders

[dunglas]

#3682 introduced support for env var defaults in Caddyfile, which is awesome! It would be nice to support default values in env.* placeholders too.

For instance, I have the following use case:

    mercure {
        # Transport to use (default to Bolt)
        transport_url {$MERCURE_TRANSPORT_URL:bolt://mercure.db}
     }

The MERCURE_TRANSPORT_URL env var may contain sensitive data (such as passwords). Using runtime env var would be a bit more secure than the current situation where the content of the password is stored in the internal config, but it's currently not possible (AFAIK) because it's not possible to set a default value for these env vars.

[mholt]

I think this is a duplicate of #1793.

As such, I'm going to close this, but your comment is duly noted and I bet it can be done without too much difficulty.