api: move highlevel functions into extension

examples/fetch_state.rs

@@ -1,5 +1,5 @@
 use env_logger;
-use mpvipc::{Error as MpvError, Mpv};
+use mpvipc::{Error as MpvError, Mpv, MpvExt};
 
 #[tokio::main]
 async fn main() -> Result<(), MpvError> {

examples/media_player.rs

@@ -1,5 +1,5 @@
 use env_logger;
-use mpvipc::{Error, Event, Mpv, MpvDataType, Property};
+use mpvipc::{Error, Event, Mpv, MpvExt, MpvDataType, Property};
 use std::io::{self, Write};
 
 fn seconds_to_hms(total: f64) -> String {