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).