Allow customization of TextAppearance in Title and Item Title with custom styles.

```Added two attributes for Title and Item Title style. Uses defaults, but can be overwritten in a custom BottomSheet.Dialog style.```

Hi, I'm using this library in my proyect. It's very nice (thanks for it), but I needed to tweak some styles. 
I added two customizable style attributes to set the text style in list items and title.

A lot more custom attributes could be added in order to further customize the looks of the Bottom Sheet, but for now I only needed those two. If you like the solution I would be glad to keep adding more custom attributes.

I hope you find it useful.