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`.

slstatus.h

@@ -5,6 +5,9 @@ const char *battery_perc(const char *);
 const char *battery_state(const char *);
 const char *battery_remaining(const char *);
 
+/* cat */
+const char *cat(const char *path);
+
 /* cpu */
 const char *cpu_freq(const char *unused);
 const char *cpu_perc(const char *unused);