Add option --invert-captures.

The new option inverts the meaning on capturing and non-capturing
parentheses: `(! ...)` becomes capturing, and `( ... )` becomes
non-capturing, including the implicit whole-regexp group, which also
becomes non-capturing.

The idea of the new option is to let the user decide which gropus should
be capturing and which are purely structural, and explicitly mark only
the capturing ones for submatch extraction.

bootstrap/lib/parse.cc

@@ -1144,13 +1144,13 @@ YYSTYPE yylval YY_INITIAL_VALUE (= yyval_default);
 
   case 13: /* primary: '(' ')'  */
 #line 77 "../lib/parse.ypp"
-               { (yyval.regexp) = ast.cap(ast.nil(NOWHERE)); }
+               { (yyval.regexp) = ast.cap(ast.nil(NOWHERE), true); }
 #line 1149 "lib/parse.cc"
     break;
 
   case 14: /* primary: '(' expr ')'  */
 #line 78 "../lib/parse.ypp"
-               { (yyval.regexp) = ast.cap((yyvsp[-1].regexp)); }
+               { (yyval.regexp) = ast.cap((yyvsp[-1].regexp), true); }
 #line 1155 "lib/parse.cc"
     break;