Allow plug_basic_auth to use an htpasswd file user repository.

Hi -- I added a couple of lines of code to provide support for configurable user repositories and implemented one based on htpasswd.  Your tests continue to pass without modifications; my tests are a copy of yours with a differently configured PrivatePlug (and they seem to pass too).  

No worries if this isn't the direction you're going with the library.  I really appreciated the example code though!