Ability to pass multiple --env-file arguments in docker-compose

[peetw]

Building on #273 it would be great if we could specify multiple --env-file arguments for docker-compose projects. Currently we can specify a single --env-file arg using <DockerComposeEnvFilePath> but often we need to specify multiple (e.g. one for static/global values and one for local/secret values).

[NCarlsonMSFT]

I've added support for a new property DockerComposeEnvFilePaths that should show up in the 18.5 timeframe.