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`.

config.def.h

@@ -18,6 +18,7 @@ static const char unknown_str[] = "n/a";
  *                                                     NULL on OpenBSD/FreeBSD
  * battery_remaining   battery remaining HH:MM         battery name (BAT0)
  *                                                     NULL on OpenBSD/FreeBSD
+ * cat                 read arbitrary file             path
  * cpu_perc            cpu usage in percent            NULL
  * cpu_freq            cpu frequency in MHz            NULL
  * datetime            date and time                   format string (%F %T)