Add analyze open documents only

[neoGeneva]

Hey, this is a pull requestion to add an AnalyzeOpenDocumentsOnly option to RoslynExtensionsOptions, which CSharpDiagnosticWorkerWithAnalyzer uses to decided if it runs full roslyn anaylizers or just the base diagnostics provided by syntaxTree.GetDiagnostics().

The main benefit is the speed of the initial analysis of all files in the solution. For large solutions this is pretty handy, for example the roslyn solution takes 18 minutes for that first pass, vs. 1 minute with AnalyzeOpenDocumentsOnly set to true.

[dnfadmin]

All CLA requirements met.

[filipw]

LGTM, thanks! - any thoughts @333fred @JoeRobich ?

[neoGeneva]

Wooo! Thanks so much for this folks, this is my first real contribution to a public repo, feels good to help out ❤️

[333fred]

@neoGeneva, would you want to submit a PR to the vscode to add the option? You'll need to:

• Add the setting to the dictionary, like this: https://github.com/OmniSharp/omnisharp-vscode/blob/master/src/omnisharp/server.ts#L390-L392
• Add the setting to the options type, with a default: https://github.com/OmniSharp/omnisharp-vscode/blob/master/src/omnisharp/options.ts#L8
• Add the setting to the package.json with a title and a description, like this: https://github.com/OmniSharp/omnisharp-vscode/blob/master/package.json#L993-L997
• Adjust this test: https://github.com/OmniSharp/omnisharp-vscode/blob/master/test/unitTests/options.test.ts#L13-L39
• Adjust this constructor: https://github.com/OmniSharp/omnisharp-vscode/blob/master/test/unitTests/Fakes/FakeOptions.ts#L9

My suggestion would be off (false) by default for this setting.

[neoGeneva]

Sweet, thanks @333fred, I've got most of it ready to go, but it got mixed up with some other stuff I was looking at, just splitting it out into its own branch so I can do a pull request a little later.