feat(cli): specify server flags via environment

[shouya]

It's often more convenient to specify the server flags via environment
than overriding the default command line. This pull request adds the
ability to specify the server flags via environment.

Currently, the following environments are supported:

Variable	Default value	Description
RSS_FUNNEL_BIND	127.0.0.1:4080	The server bind address
RSS_FUNNEL_INSPECTOR_UI	true	Whether or not to serve the inspector UI
RSS_FUNNEL_WATCH	false	Whether or not to watch config file change and automatically reload
RSS_FUNNEL_CONFIG	unspecified	The location of the config file.

Note: if you specify the options both from environment and command
line, the command line options will take precedence.