Added support for relative volume (v+ and v-)

mml2esf/doc/echomml.txt

@@ -150,6 +150,12 @@ put spaces *in the middle* of a command.
 
       Decrease or increase the volume by 1, respectively.
 
+   v+ v-
+
+      Followed by a number, increase or decrease the volume by that amount,
+      respectively. Like doing multiple ( or ) in a row, except only one
+      event is generated (which is better).
+
    p
 
       Followed by a number between 0 and 3, changes the panning of the

mml2esf/tool/mml.c

@@ -822,12 +822,32 @@ static int parse_commands(const char *data, unsigned channel, unsigned line)
          if (channel == 0x10) goto noctrl;
          data++;
 
+         // Check if it's relative
+         int sign = 0;
+         if (*data == '+') {
+            sign = 1;
+            data++;
+         } else if (*data == '-') {
+            sign = -1;
+            data++;
+         }
+
          // Get volume
          int volume = parse_number(&data);
          if (volume == -1) {
             fprintf(stderr, "Error[%u]: missing new volume\n", line);
             return -1;
          }
+         switch (sign) {
+            case 0:
+               break;
+            case 1:
+               volume = chanstat[channel].volume + volume;
+               break;
+            case -1:
+               volume = chanstat[channel].volume - volume;
+               break;
+         }
          if (volume < 0 || volume > 15) {
             fprintf(stderr, "Error[%u]: invalid volume value\n", line);
             return -1;
@@ -907,10 +927,15 @@ static int parse_commands(const char *data, unsigned channel, unsigned line)
       }
 
       // Whitespace? (skip it)
-      else if (*data == ' ' || (*data >= 0x08 && *data <= 0x0D)) {
+      else if (is_whitespace(*data)) {
          data++;
       }
 
+      // Comment?
+      else if (*data == ';') {
+         break;
+      }
+
       // Unknown or unimplemented command
       else {
          fprintf(stderr, "Error[%u]: invalid command \"%c\"\n", line, *data);