Make formatting columns in grid easier, once model is linked

[romaninsh]

It should be simpler to add or change Column objects for columns. By default we are re-using same column for all columns, so this shouldn't be difficult.

$grid->setModel(new User($db));
$grid->setColumn('user', ['Link', 'mypage']);

Please give me feedback on what method should be called:

• addFormatter
• setFormatter
• format
• setColumn
• changeColumn

Also in some cases you want to change format in others - add extra formatter. See #67, #92