Add optional feature to use brackets instead of parens for 6502 indirect addressing.

src/ca65/feature.c

@@ -64,6 +64,7 @@ static const char* FeatureKeys[FEAT_COUNT] = {
     "force_range",
     "underline_in_numbers",
     "addrsize",
+    "bracket_as_indirect",
 };
 
 
@@ -121,6 +122,7 @@ feature_t SetFeature (const StrBuf* Key)
         case FEAT_FORCE_RANGE:                ForceRange        = 1;    break;
         case FEAT_UNDERLINE_IN_NUMBERS:       UnderlineInNumbers= 1;    break;
         case FEAT_ADDRSIZE:                   AddrSize          = 1;    break;
+        case FEAT_BRACKET_AS_INDIRECT:        BracketAsIndirect = 1;    break;
         default:                         /* Keep gcc silent */          break;
     }