Add support for multiple users

[misterbisson]

#1 implements a scheme to set a single user/password pair, but the mechanism is not extensible to support multiple u/p pairs for an arbitrary number of users.

An alternate approach might be to generate the user/password pair from a directory of key/value pairs in Consul. The implementation now sets the single user/pass pair as a key/value pair in a proposed directory, but does not implement a mechanism to watch that directory for changes or generate a new htpasswd file from it.