The .editorconfig should be updated to support the code style options made available in VS2017

[tannergooding]

Currently we have a set of code style guidelines here: https://github.com/dotnet/corefx/blob/master/Documentation/coding-guidelines/coding-style.md.

However, we don't have a good way to enforce this (aside from manual code reviews and occasionally running https://github.com/dotnet/codeformatter) when users have their VS options configured away from the defaults.

VS2017 added support for specifying a majority of these options via the .editorconfig file:

• add the first set of editor options for visual studio roslyn#15020
• editorconfig codestyle options roslyn#15029
• editor config naming style roslyn#15065

We should update the repositories .editorconfig to have a set of style options matching those specified in our coding-style documentation.