support strange joins

[MarinPostma]

Sqite has a rather flexible syntax for joins, and the parser should support them. This PR alleviate the restriction on joins, and make them more flexible.

see: https://www.sqlite.org/lang_select.html#strange_join_names

[psarna]

@MarinPostma #23 (comment) it looks like weird joins are fine on their own, looks like we hit another kind of issue

[MarinPostma]

No they aren't, for example, this is not supported by the parser, but should:
SELECT * FROM foo LEFT NATURAL JOIN bar

[gwenn]

I am going to try fixing #23 first.
https://www.sqlite.org/lang_select.html#strange_join_names

Remember: you can use these non-standard join types but you ought not. Stick to using standard JOIN syntax for portability with other SQL database engines.

And I am not sure we want to support these non-standard join types (like Double-Quoted String).

[gwenn]

It seems that the actual rules are here:
https://github.com/sqlite/sqlite/blob/80511f32f7e71062026edd471913ef0455563964/src/select.c#L197-L257

The only restrictions on the join type name are:

• "INNER" cannot appear together with "OUTER", "LEFT", "RIGHT",
  or "FULL".

• "CROSS" cannot appear together with "OUTER", "LEFT", "RIGHT,
  or "FULL".

• If "OUTER" is present then there must also be one of
  "LEFT", "RIGHT", or "FULL"

[MarinPostma]

I'll implement that 👍

[gwenn]

See #37