Added support for chainable protocols eg: <protocol>:<value>|<protocol>:

Thought this might be a neat feature to add as an alternative approach registering multiple handlers for the same protocol.

I'm presently using it as follows:

```
{
    "params":"file:path/to/config/file|filter:.options .params" 
}
```

1. Retrieve JSON file
2. Filter values ( note I use jsonselect for this )

Consumers of `shortstop` can then setup default handlers ( as in my case ) which can then be used independently or as a chain.