added configureFlatCellWithColor

currently, if an app is using a custom cell class, it's impossible to use `configureFlatCellWithColor`, as it can only generate standard `UITableViewCell` classes.

this adds a `configureFlatCellWithColor` function which can be used on existing sub classes of `UITableViewCell`.

for example:

```

cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
[cell configureFlatCellWithColor:[UIColor greenSeaColor] selectedColor:[UIColor cloudsColor]];

```