1.4.0

[bvaughn]

Resolves #100, #104, #106, #111

• List and Grid components now "overscan" (pre-render) in both directions when scrolling is not active. When scrolling is in progress, cells are only pre-rendered in the direction being scrolled. This change has been made in an effort to reduce visible flicker when scrolling starts without adding additional overhead during scroll (which is the most performance sensitive time).
• Grid components now support separate overscanColumnsCount and overscanRowsCount props. Legacy overscanCount prop will continue to work, but with a deprecation warning in DEV mode.
• Replaced setTimeout with requestAnimationFrame based timer, to avoid starvation issue for isScrolling reset.
• Renamed List and Grid innerTagName and outerTagName props to innerElementType and outerElementType to formalize support for attaching arbitrary props (e.g. test ids) to List and Grid inner and outer DOM elements. Legacy innerTagName and outerTagName props will continue to work, but with a deprecation warning in DEV mode.
• List re-renders items if direction prop changes.