New component: cat

Generically reads an arbitrary file natively.

Saves a few layers of execution in comparison to using `run_command`
with an argument like `cat ./file`.

README

@@ -7,6 +7,7 @@ slstatus is a suckless status monitor for window managers that use WM_NAME
 Features
 --------
 - Battery percentage/state/time left
+- Cat (read file)
 - CPU usage
 - CPU frequency
 - Custom shell commands