Add ?, .[]?, and .. operators

Make XPath-like `//a/b` recursive structure traversal easier in jq,
which then becomes:

    ..|.a?.b?

The `?` operator suppresses errors about . not being an array or object.
The `..` operator is equivalent to calling the new `recurse_down`
built-in, which in turn is equivalent to

    recurse(.[]?)

Note that `..a` is not supported; neither is `...a`.  That could be add
added, but it doesn't seem worth the trouble of saving the need to type
a '|'.

lexer.l

@@ -61,7 +61,8 @@ struct lexer_param;
 "//=" { return SETDEFINEDOR; }
 "<=" { return LESSEQ; }
 ">=" { return GREATEREQ; }
-"."|"="|";"|","|":"|"|"|"+"|"-"|"*"|"/"|"%"|"\$"|"<"|">" { return yytext[0];}
+".." { return REC; }
+"."|"?"|"="|";"|","|":"|"|"|"+"|"-"|"*"|"/"|"%"|"\$"|"<"|">" { return yytext[0];}
 
 "["|"{"|"(" {
   return enter(yytext[0], YY_START, yyscanner);