Adding `--config` parameter to support specifying configuration files

[julianosam]

Small change to support specifying JSON config files via --config argument. This is useful if your project has more than one setup that needs to be used in different situations. For example, one config file containing proxy configuration pointing to real test servers and another one pointing to your local box. Easy switching with this option.

[littleclown]

Can I directly specify the configuration defined in .vscode/settings.json? Sometimes I don’t want to open vscode, but want to start the configured live-server server directly in the terminal.