Make --logpath work again

Also add a new option "log-file" to config file and command line, it
doesn the same thing as --logpath.

--logpath was never documented, and "log-file" is more consistent with
the naming of options.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

compton.sample.conf

@@ -3,6 +3,8 @@ shadow = true;
 shadow-radius = 7;
 shadow-offset-x = -7;
 shadow-offset-y = -7;
+log-level = "warn";
+# log-file = "/path/to/your/log/file";
 # shadow-opacity = 0.7;
 # shadow-red = 0.0;
 # shadow-green = 0.0;