Change debug color from white to blue

This makes it much more readable if the terminal background is a light color.

Fixes issue #138

Author: ifreund

src/fmt/writer/termcolor/extern_impl.rs

@@ -52,7 +52,7 @@ impl Formatter {
         let mut level_style = self.style();
         match level {
             Level::Trace => level_style.set_color(Color::Black).set_intense(true),
-            Level::Debug => level_style.set_color(Color::White),
+            Level::Debug => level_style.set_color(Color::Blue),
             Level::Info => level_style.set_color(Color::Green),
             Level::Warn => level_style.set_color(Color::Yellow),
             Level::Error => level_style.set_color(Color::Red).set_bold(true),