Allow developer to configure auto focus on the grid (addresses #813)

[supamanda]

Description

A PR to allow developer to configure whether the grid should steal focus when the browser is focused on the body. Addresses #813

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/adazzle/react-data-grid/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[ ] Bugfix
[x ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here)
The grid steals focus when the browser is focused on the body, see #813

What is the new behavior?
Allow developer to configure whether grid should autofocus when browser is focused on the body

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[x ] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

[coveralls]

Changes Unknown when pulling c0ca889 on supamanda:ah-configure-auto-focus into ** on adazzle:master**.

[supamanda]

There was a problem with this PR because it's contaminated with #843 - have opened #845 instead