v1.0

- `maybe_placeholders` is now True by default

- Renamed TraditionalLexer to BasicLexer, and 'standard' lexer option to 'basic'

- Default priority is now 0, for both terminals and rules (used to be 1 for terminals)

- Discard mechanism is now done by returning Discard, instead of raising it as an exception.

- `use_accepts` in `UnexpectedInput.match_examples()` is now True by default

- `v_args(meta=True)` now gives meta as the first argument. i.e. `(meta, children)`
