Add ability to set Hook.inputs and Hook.outputs

[Marak]

There should be two new properties on hook resource, Hook.inputs, and Hook.outputs.

Both properties are arrays of strings which contain names of hooks.

inputs are piped left to right into the hook. outputs are piped left to right from the output of the hook.

The logic for this should be easy since we are using all streams and have access to .pipe().

[Marak]

I attempted a first version of this and ended up doubling back and refactoring out a few modules from hook.io core into separate projects.

This should be easier to implement now.

If anyone wants to take a shot at it, please do. If not, I will most likely attempting again next month.

[Marak]

Once the additional refactors from #136 are completed, this should be even easier to implement.

[Marak]

We've not got the ability to chain services as middlewares.

This is really cool. Any service can act as an input or middleware.

Will be deployed soon.