Add gridRowStyle and gridColumnStyle props

[pdpino]

Fixes #43, fixes #138, fixes #156

• Add gridRowStyle and gridColumnStyle props, to modify grid horizontal and vertical lines (separated). Usage:

  gridRowStyle={{ borderTopWidth: <width>, borderColor: <color> }} // horizontal lines width and color
  gridColumnStyle={{ borderLeftWidth: <width>, borderColor: <color> }} // vertical lines width and color

• I removed an extra <View/> element, for simplicity

I'm not sure the props I chose are the best option, lmk if you think there are better options

[pdpino]

Other prop options I thought:

1. two props gridLineWidth and gridLineColor, but this does not discriminate between horizontal and vertical lines (issue It is possible to show only horizontal grid lines and hide vertical grid lines and also make size of lines bigger? #156)
2. four props horizontalLineWidth, verticalLineWidth, horizontalLineColor, verticalLineColor, but they are too many props (?)
3. (I chose this) two props gridRowStyle and gridColumnStyle, allow passing more styles if necessary (in the future), though users could break stuff (if passing flex: 2, for example)

[hoangnm]

Thank @pdpino , I think this is a good option. LGTM.