Allow replacing each of the many sets of separate foo-{fg,bg,attr}

options with a single foo-style option. For example:

    set -g status-fg yellow
    set -g status-bg red
    set -g status-attr blink

Becomes:

    set -g status-style fg=yellow,bg=red,blink

The -a flag to set can be used to add to rather than replace a style. So:

    set -g status-bg red

Becomes:

    set -ag status-style bg=red

Currently this is fully backwards compatible (all *-{fg,bg,attr} options
remain) but the plan is to deprecate them over time.

From Tiago Cunha.

Makefile

@@ -113,6 +113,7 @@ SRCS=	arguments.c \
 	session.c \
 	signal.c \
 	status.c \
+	style.c \
 	tmux.c \
 	tty-acs.c \
 	tty-keys.c \