Add option for Redis key prefixes.

[jack-karamanian]

This PR allows user specified text to be inserted between keys.

This is enabled through the plugins options, as such:

'Plugin::Session' => {
    key_prefix => 'example'
}

This option will produce Redis keys as follows:

session:example:971331427b83f4375988c3373285b7261a7c2c4d

expires:example:971331427b83f4375988c3373285b7261a7c2c4d

This feature is useful to categorize sessions within a single Redis database.