React Native Section Alphabet List Versions Save

Add support for overriding the style of the flat list container.

Add support for overriding the style of the list index container.

• Add support for rendering custom letter indexes using renderCustomIndexLetter prop
• Add support for styling the letter index using indexLetterStyle prop
• Add support for styling the letter index container using indexLetterContainerStyle prop
• Remove indexLetterColor prop - you can now change the colour of the letter index using indexLetterStyle

Add uncategorizedAtTop prop. By default, its value is false. If true, the uncategorised items (the # section) is sorted to the top of the list.