tmux: set TMUX_VERSION variable as X*100 + Y

Signed-off-by: Alejandro Mery <amery@geeks.cl>

files/.tmux.conf

@@ -1,3 +1,10 @@
+# load TMUX_VERSION
+run-shell "\
+	echo #{version} | sed -e 's|\\.| |g' -e 's|[a-z]*$||' |\
+	while read x y; do\
+		tmux set-environment -g TMUX_VERSION \$((x * 100 + y));\
+	done"
+
 # same prefix as `screen`
 set -g prefix C-a
 unbind C-b