Added support for custom button image

[Mikadifo]

Using this library I realized that the iOS store button image link is always breaking because of the strict apple guidelines. Instead of having the developers that use this library patch it with patch-package for example, I believe it's better to have some sort of fallback property that allows them to use any image they want.

Changes:

• I made the 'store' property optional but it will default to "android" if not specified
• Added new property 'imgSrc' which is an optional string.
• In the render method if 'imgSrc' is not provided, it will render the specified 'store'
• If neither 'store' nor 'imgSrc' are provided, it will render the "android" store button
• New lib directory generated using yarn run build

This can also help devs use buttons in languages other than English.