Typed emission

[Stebalien]

Currently, the "response emitter" has a single Emit function that magically determines the right thing to do. Unfortunately, this is really magical:

1. We have a special "single" wrapper to inform HTTP that we're emitting a single value.
2. If the user "emits" a reader, we copy it as a stream of bytes.
3. If the user emits normal values (not Single, we emit a stream of values).

This is getting better with the next iteration of the commands lib (this list used to include errors) but it could be improved.

Solutions:

1. Have different command types. While the nicest from a type safety standpoint, this may not be the friendliest option.
2. Apply a type-switch to the command's Run (and PostRun) functions. Personally, this is the one I'd vote for.

In the second case, we'd allow the following Run signatures:

// Writes a stream
type StreamFunc func(req *Request, resp io.Writer, env Environment) error

// Streams items
//
// We can keep this "magical" for now but eventually remove the magic and force users to use the other variants.
type EmitterFunc func(req *Request, resp ResponseEmitter, env Environment) error

// Sends a single item
type SingleFunc func(req *Request, env Environment) (interface{}, error)

Thoughts? This should make sending HTTP responses simpler and will likely improve user experience.