This allows users who do not wish to use bind.WithFlag() to still take
advantage of bind's magical listener selection logic.

Fixes #68.

Expose an additional function, bind.WithFlag(), which allows callers to
use the previously-default "global flag" mode.

This change allows the bind string parser (etc.) to be used without
unwanted side effects. The behavior of the top-level "goji" package has
not been changed.

Fixes #47.

This eliminates the race condition mentioned in a54c913a by forbidding
duplicate binds to the same socket (well, at least in the sense that
attempting to do so will *always* result in an error instead of
nondeterministically resulting in an error).

Goji now honors two environment variables: GOJI_BIND (which accepts the
full "bind" flag syntax) and PORT (which is treated as a TCP port).

Ref #20.

Package bind provides a convenient syntax for binding to sockets, as well as a
fair bit of magic to automatically Do The Right Thing in both development and
production.