Lower default osu hp drain

DDR stepcharts are harder due to how they are played. That means that a
hard DDR chart will have much less notes than a hard mania chart. Since
osu does not know that, giving it a (mania typical) hp drain will be
extremely hard with a DDR chart.

src/converter/ddr2osu.rs

@@ -57,7 +57,7 @@ pub struct Config {
     #[clap(
         long = "hp",
         about = "Range of HP drain (beginner:challenge)",
-        default_value = "4:8"
+        default_value = "2:4"
     )]
     pub hp_drain: ConfigRange,
     #[clap(