Implement variables in configfile.

This implements ticket #42.

Syntax is "set $key value". All further instances of $key will be
replaced with value before parsing each line of the configfile.

include/config.h

@@ -15,6 +15,8 @@
 #ifndef _CONFIG_H
 #define _CONFIG_H
 
+#include "queue.h"
+
 typedef struct Config Config;
 extern Config config;
 
@@ -24,6 +26,13 @@ struct Colortriple {
         uint32_t text;
 };
 
+struct Variable {
+        char *key;
+        char *value;
+
+        SLIST_ENTRY(Variable) variables;
+};
+
 struct Config {
         const char *terminal;
         const char *font;