Add support for onPress on Bubble

[charleslouis]

• Callback when a message bubble is pressed;

This is helpful to implement rich cards chats such as in the following scenario in a chatbot :
1- a user receives a set of bubbles as options to click
2- the user presses the bubble and this sends a pre-written question

An example of implementation could be :

    // restyle the bubbles based on user or richcards
    renderBubble = (props) => {
        let wrapperStyle = {
            left: {
                padding: 5,
                backgroundColor: '#40216F',
            },
            right: {
                padding: 5,
                backgroundColor: '#2678D7',
            }
        };
        let textStyle = {
            left: {
                color: '#ffffff',
                fontSize: 16
            },
            right: {
                color: '#ffffff',
                fontSize: 16,
            }
        };

        let wrapperStyleQuestionBubble = {
          backgroundColor: '#ffffff',
          borderColor: '#40216F',
          borderWidth: 3,
          borderRadius: 10,
        }

        let wrapperStyleQuestionBubbleTextColor = '#40216F';

        let TouchableOpacityBubbleStyle = {
            borderColor: '#FF33AA',
            borderWidth: 3,
            backgroundColor: '#33AAFF',
        }

        if (props.currentMessage.richCard) {

            wrapperStyle.left = wrapperStyleQuestionBubble;
            textStyle.left.color = wrapperStyleQuestionBubbleTextColor;

            return (
                <TouchableOpacity>
                    <Bubble
                        {...props}
                        onPress={() => props.sendRichCard(props.currentMessage)}
                        wrapperStyle={wrapperStyle}
                        textStyle={textStyle}/>
                </TouchableOpacity>
            );
        }

        return (
            <Bubble
                {...props}
                wrapperStyle={wrapperStyle}
                textStyle={textStyle}/>
        );
    }

[charleslouis]

And thank you for this great module by the way !

[Faolain]

@charleslouis can you please fix the linting error

'onPress' is missing in props validation react/prop-types

And resubmit the PR? Also how does this differ from this #826 ?