This commit introduces TLS support, provided by `rustls` and a fork of
`hyper-rustls`. TLS support is enabled via the `tls` feature and
activated when the `tls` configuration parameter is set. A new
`hello_tls` example illustrates its usage.
This commit also introduces more robust and complete configuration
settings via environment variables. In particular, quoted string,
array, and table (dictionaries) based configuration parameters can now
be set via environment variables.
Resolves#28.
