feat: Add support for block-specific icons

Icons can now be specified on a per-block basis. Closes #64.

src/main.c

@@ -128,7 +128,8 @@ int main(const int argc, const char *const argv[]) {
         return 1;
     }
 
-#define BLOCK(command, interval, signal) block_new(command, interval, signal),
+#define BLOCK(icon, command, interval, signal) \
+    block_new(icon, command, interval, signal),
     block blocks[BLOCK_COUNT] = {BLOCKS(BLOCK)};
 #undef BLOCK
     const unsigned short block_count = LEN(blocks);