Checkupdates & Divider Functions

- Add checkupdates function to count number of available updates
- Add divider function to print divider symbol as argument
- Update README with these modifications

components/check_updates.c

@@ -0,0 +1,12 @@
+/* See LICENSE file for copyright and license details. */
+#include <stdio.h>
+#include <string.h>
+
+#include "../slstatus.h"
+
+const char *
+check_updates(void)
+{
+    const char *cmd = "checkupdates | wc -l";
+    return run_command(cmd);
+}