Add "using" keyword to C#

[breyed]

No description provided.

[Coeur]

Note that the reference list of C# keywords is available at https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/

I've been through the list, and it's actually missing three keywords:

• explicit
• namespace
• using

[Coeur]

Good, but incomplete: please add explicit and namespace too.

[breyed]

I added explicit and namespace. Thanks for diffing through the keyword list.

[Coeur]

@mikesamuel, may you merge this one as well?

[mikesamuel]

Please make the changes to js-modules/prettify.js instead and then run

$ npm install && npm run grunt