Investigate Grid component

[bvaughn]

Create a new Grid component that supports virtualization of both rows (vertical) and columns (horizontal). Interface may look something like:

<Grid
  columnsCount={}
  columnWidth={index => {}}
  rowsCount={}
  rowHeight={index => {}}
  renderCell={({ columnIndex, rowIndex }) => {}}
  scrollToColumn={}
  scrollToRow={}
/>

Blockers

• Refactor VirtualScroll's vertical virtualization functions into a helper utility.

[bvaughn]

Related to issue #6