Improved seek command to allow more units (#682)

* `seek` command accepts fancy duration

* Use default formatting

* Better var naming

* Documented `seek` command

Cargo.toml

@@ -47,6 +47,7 @@ ioctl-rs = { version = "0.2", optional = true }
 serde_cbor = "0.11.2"
 pancurses = { version = "0.17.0", features = ["win32"], optional = true }
 libc = "0.2.111"
+parse_duration = "2.1.1"
 
 [dependencies.rspotify]
 version = "0.11.3"