Add onPressEnter for select component

What problem does this feature solve?

When we use Select with ‘combobox’ mode, we usually do some thing when user press Enter, like do a search action. Hope to support this feature.

What does the proposed API look like?

<Select onPressEnter={this.handlePressEnter}>{options}</Select>

[afc163]

There are two workarounds for now.

1. AutoComplete with input[onKeyDown]: https://codepen.io/afc163/pen/VWRqVo?editors=001
2. Select with onKeyDown in wrapper: https://codepen.io/afc163/pen/NgJeep?editors=001 like @yesmeck said.

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And we still need to fix Select[combobox][onKeyDown].

[yesmeck]

I think it’s ok to fire on every keyDown event and in all modes. People who want Enter event just check e.keyCode.