feat: Add Custom Font Weight Styling

[abifdhllh]

Overview

Added custom font styling for font-weight to fix issue #130

Test Plan

I Added fontWeight property on fontStyle & activeFontStyle props. Here is the example:

<SegmentedControl
  values={['One', 'Two']}
  selectedIndex={this.state.selectedIndex}
  onChange={(event) => {
    this.setState({selectedIndex: event.nativeEvent.selectedSegmentIndex});
  }}
  activeFontStyle={{
    fontWeight: '500'
  }}
  fontStyle={{
    fontWeight: '200'
  }}
/>

[Naturalclar]

@abifdhllh thanks!