Add initial WP::request() & friends API method with unslashing, optional default, optional sanitization.

[dd32]

WIP idea.

http://example.org/?foo[]=bar&baz[bar]=3

// Sub-key access
$value = WP::get( [ 'baz', 'bar' ], 1, 'integer'  );
var_dump( $value ); // int 3

// Sub key => Default
$value = WP::get( [ 'foo', 'unset', 'value' ], 1, 'integer'  );
var_dump( $value ); // int 1

// Something that's actually kind meh.. "foo[]=bar" => float 1 ??
$value = WP::get( 'foo', false, 'number'  );
var_dump( $value ); // float 1 - not an array.

// Something that's better than worst case, "foo[]=.." => string val "Array"
$value = WP::get( 'foo', 'default value here', 'string'  );
var_dump( $value ); // string 'Array'