Add Reactor API

[eNV25]

From #22.

The last comment mentions using an interface in the old pull request. I liked the idea and I want to implement it.

Here's how I'm imagining it would look like:

// Reactor represents an MPD connection that reacts to events.
type Reactor struct{ ... }

// NewReactor connects to MPD and reacts to events. Set handlers and other options
// by passing ReactorOptions.
func NewReactor(network, addr, password string, options ...ReactorOption) *Reactor

// Subsystems restricts subsystems watched for.
func (r *Reactor) Subsystems(subsystems ...string)

// Interrupt stops watching for event temporarily and runs the interrupt handler with args.
func (r *Reactor) Interrupt(args ...interface{})

// ReactorOption
type ReactorOption func(r *Reactor)

// ReactorSubsystems restricts the subsystems watched for.
func ReactorSubsystems(subsystems ...string) ReactorOption

// ReactorEventHandler is run for each change in MPD. The first argument is the subsystem
// that changed. See the list of subsystems:
// https://mpd.readthedocs.io/en/stable/protocol.html#command-idle
func ReactorEventHandler(f func(c *Client, subsystem string)) ReactorOption

func ReactorErrorHandler(f func(c *Client, err error)) ReactorOption

func ReactorInterruptHandler(f func(c *Client, args ...interface{})) ReactorOption

Instead of an interface, this uses the functional options pattern. This makes the API easier to use and makes any or all functionality optional.

This also adds an interrupt system, which allows using Client for external events. Interrupt will simply call noidle and send args on a channel.

We could also add ReactorOptions that send on a channel, so Watcher could be implemented in terms of Reactor.

[eNV25]

#74