unpacking a tuple argument is invalid in Python 3.

    >>> def foo(a, (b, c)):
      File "<stdin>", line 1
        def foo(a, (b, c)):
                   ^
    SyntaxError: invalid syntax

Fixes #403

Closes #305.

I interpreted "the column where the token ends" to mean "a pointer
to the last character in the token", which would be the same as
the start column for a single-character token.

However, that's not what lark actually reports. Reword to clarify
this.

https://github.com/lark-parser/lark/issues/240

* Removed the scanless parsing feature (dynamic lexing is king)
* Default LALR lexer is now contextual

- @erezsh added the new Lark reference and some other things using the
  current .g extension, so I wanted to resolve them for PR #117
- Renamed lark.g to lark.lark.
- Changed lark_grammar.py to use .lark file extensions.
- Changed urls that used .g to use .lark.