Piccolo

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* Exposing ``index_type`` in the ``Column`` constructor.
* Fixing a typo with ``start_connection_pool` and ``close_connection_pool`` -
thanks to paolodina for finding this.
* Fixing a typo in the ``PostgresEngine`` docs - courtesy of paolodina.

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Fixing a bug with ``SchemaSnapshot`` if column types were changed in migrations
- the snapshot didn't reflect the changes.

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Added ``TableConfig``, which allows more fine grained control over how the
UI behaves for a given ``Table``. Currently it allows you to specify which
columns are visible on the list page, but more options will be added in the
future. Courtesy sinisaos.

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The Postgres extensions that ``PostgresEngine`` tries to enable at startup
can now be configured.

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* Fixed a bug with ``MyTable.column != None``
* Added ``is_null`` and ``is_not_null`` methods, to avoid linting issues when
comparing with None.

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* Added ``WhereRaw``, so raw SQL can be used in where clauses.
* ``piccolo shell run`` now uses syntax highlighting - courtesy of Fingel.

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