Add border options to `selectionBackground`

[evandrocoan]

• VSCode Version: 1.14.1:
• OS Version: Windows 10 build 15063

Add border options to selectionBackground and findMatchBackground. They would be called like selectionBorder and findMatchBorder. You cannot find them on:

1. https://code.visualstudio.com/docs/getstarted/theme-color-reference

Currently the selection has no border:

Would be awesome if it has borders like Sublime Text selections:

Reproduces without extensions: Yes

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Related issues:

1. additional options for editor.selectionHighlight  #8050 additional options for editor.selectionHighlight

[ro31337]

+1