Make `main.rs` responsible for only the `main()` function and command
line option handling. Move the actual application code to `lib.rs` for
clearer separation of concerns.

Previously, errors would be returned immediately by the `?`/`try!`s.
Handle errors directly instead of returning them. Otherwise, the stream
listener would terminate, exiting the program, and forcing users to
restart it in order to restore functionality.

When printing errors to STDERR, ensure we're writing newlines to
separate error messages.

Using `expect` seems nicer here because it allows us to provide a more
meaningful error message.

Parse the URL coming in from the TCP stream to check that it's a valid
URL. This allows us to fail early and not try to open strings that are
not URLs. Leverage the "url" crate to do the parsing.

Return a `Box<Error>` from `open_stream()` to enable us to return either
an `io` or a `Url` error in our `Result`.

Remove the `trim_right()` call on `url` since it's now a `Url` type
instead of a `String`, and as a result of parsing to a `Url`, it no
longer has the problematic trailing newline.

Instead get these from function arguments, which in turn get fed from
the command line arguments passed by the user.

Remove the magic number and replace it with a named value.

Provide users with an option to change the default port the program
listens on.

Instead of checking for the presence of the `-b` option before setting
it, just use an `Option` match since `opt_str` returns an
`Option<String>`.