Allow underscores in IDENT tokens. Fixes #3.

IDENT syntax now includes ASCII letters and underscores, so
'.foo_bar' now works. Non-ASCII letters won't work in IDENT
tokens (it's impossible to tell which non-ascii characters are
"letters" without full unicode tables), so '.données' is still
a syntax error (the workaround is '.["données"]', since you can
put anything you like in a string).

lexer.l

@@ -107,7 +107,7 @@
 }
   
 
-[[:alnum:]]+  { yylval->literal = jv_string(yytext); return IDENT;}
+[a-zA-Z_][a-zA-Z_0-9]*  { yylval->literal = jv_string(yytext); return IDENT;}
 
 [ \n\t]+  {}