Set message-style to also be default powerline style.

Like `status-style`, which sets the color scheme of the entire status line (added in #362), `message-style` sets the color scheme of the command/prompt mode (i.e. when you press `Prefix` + `:`).

Here are some screenshots:
TODO add in PR

Author: izzygomez

main.tmux

@@ -16,6 +16,7 @@ tmux set-option -g status "$TMUX_POWERLINE_STATUS_VISIBILITY"
 tmux set-option -g status-interval "$TMUX_POWERLINE_STATUS_INTERVAL"
 tmux set-option -g status-justify "$TMUX_POWERLINE_STATUS_JUSTIFICATION"
 tmux set-option -g status-style "$TMUX_POWERLINE_STATUS_STYLE"
+tmux set-option -g message-style "$TMUX_POWERLINE_STATUS_STYLE"
 
 tmux set-option -g status-left-length $TMUX_POWERLINE_STATUS_LEFT_LENGTH
 tmux set-option -g status-right-length $TMUX_POWERLINE_STATUS_RIGHT_LENGTH