Lex and parse .foo better.

'.as' is now valid, '. foo' is now invalid. See #141.

lexer.l

@@ -108,6 +108,7 @@ struct lexer_param;
   
 
 [a-zA-Z_][a-zA-Z_0-9]*  { yylval->literal = jv_string(yytext); return IDENT;}
+\.[a-zA-Z_][a-zA-Z_0-9]*  { yylval->literal = jv_string(yytext+1); return FIELD;}
 
 [ \n\t]+  {}