add key prefix option

[SerjoPepper]

I use prerender for huge amount of pages and need a prefix to avoid naming collisions with other plugins and tools. For example 'http://localhost/catalog/1' move to 'MY_PREFIX:http://localhost/catalog/1'

[JonathanBennett]

I'll have a look at this and shouldn't take long at all. If you need this quickly, a pull request would be appreciated? 👍