hitSlop added to left icon

[zeritte]

this PR is related to: #91
It basically adds hitSlop support to left navbar icon. hitSlop is something recommended by react-native-community, helps developers to improve user experience. (https://facebook.github.io/react-native/docs/improvingux#make-tappable-areas-larger)
example usage:

<NavBar
    style={styles.navbar}
    right={this.renderRight()}
    leftHitSlop={{ top: 30, bottom: 30, left: 30, right: 30 }}
    leftStyle={{ paddingVertical: -12, flex: 0.3 }}
    leftIconColor={theme.COLORS.WHITE}
    titleStyle={styles.title}
    onLeftPress={this.handleLeftPress}
/>

[palingheorghe]

Hi @zeritte ! Nice job! We'll add this when we merge everything for v0.6 into master! Thanks!