Please add support for multiple users

[MikeKemmerer]

Currently, in order to support multiple users, it is necessary to implement multiple instances of the plugin on multiple ports or set up a proxy in front of logstash that would handle the authentication, but adds a layer of complexity. If the user and password parameters were combined into a single hash where the key is the user and the value is the password, auth_token can become array of tokens. Then, validates_token could look for the token as an element in auth_token using the include method.

This would greatly simplify our implementation.

I would attempt the pull request myself, but my ruby skills are novice at best.

[andsel]

Hi @MikeKemmerer thanks to raise this, fell free to come up with a PR, it's welcome will be reviewed