feature: class based filters

[ryangjchandler]

This pull request adds support for basic class based filters. This means you can extract a filter into a reusable class that could be applied to multiple resources.

Here's what the class looks like:

class NameFilter extends Filter
{
    protected function setUp()
    {
        $this->name('Name Like a');
    }

    public function apply($query)
    {
        return $query->where('name', 'LIKE', 'a%');
    }
}

The Filter class now has a setUp() method that can be used by class-based filters to set some default arguments. The $name parameter for ::make() and __construct() is also now nullable so class-based filters don't need to provide it but they still can if they don't want to define the name as $name or using $this->name() in setUp().

This pull request also opens up the scope for future enhancements, such as parameter based filters where you could define the fields for the filter / options inside of a class.

A good example of a reusable filter is a ActiveFilter where multiple resources have a common column and column values.