Roadmap for V2.0

Here is the wishlist I'd like to have for the next major version:

* [x] Don't use `Grid` for table header
* [x] Replace `react-virtualized` with `react-window` #12
* [x] remove deprecated lifecycles #10
* [x] Dynamic row heights #170
* [ ] Make virtualization configurable
* [ ] A11y
* [ ] UT
* [ ] plugin support
* [ ] rewrite with TypeScript
* [ ] support custom children key

Breaking changes:
* [ ] use `rollup` to bundle package
* [ ] omit placeholders in `rowRenderer`
* [ ] rename `tagName` to `as` or `elementType`
* [ ] rename `scrollToPosition` to `scrollTo`
* [ ] rename `headerRenderer` to `headerRowRenderer`
* [ ] rename `headerRenderer` for `Column` to `headerCellRenderer`
* [ ] move `AutoResizer` to a separate package

Nice to have:
* [ ] better size calculation to support cell with borders #149