Configurable buffer size for input http streams

## Feature request

Currently, when MPD plays radio streams via http(s), it seems to default to pretty large buffer (around 2s), which causes pretty large latency.

Any chance this could be configurable or maybe even ability to set it per-URL (e.g. `mpd add http://my-stream/stream.mp3?buffer=500`)? Something similar to VLC's `:network-caching` property.
