config: make web application port and hostname configurable

[robvdl]

Make web application port and hostname configurable.

• SAMBAL_HOST sets the hostname, defaulting to 127.0.0.1
• SAMBAL_PORT sets the port, defaulting to 8000

The default host shouldn't be set to 0.0.0.0, because serving on all interfaces by default isn't the best for security.

But this makes it possible for the user to set it to 0.0.0.0, or a different port.