Support loading credentials from io.Reader

[pschultz]

Add the NewFromReader constructor and ReloadFromReader method for
*HtpasswdFile.

This makes it easier for users that receive credentials from sources
other than a file, because they don't have to create temporary files:

htp, _ := htpasswd.NewFromReader(strings.NewReader("foo:{PLAIN}bar"), htpasswd.DefaultSystems, nil)
fmt.Println(htp.Match("foo", "bar")) // Output: true

_ = htp.ReloadFromReader(strings.NewReader("foo:{PLAIN}baz"), nil)
fmt.Println(htp.Match("foo", "bar")) // Output: false