Multi Root Workspace Support

## Environment data
`dotnet --info` output:

```
.NET Command Line Tools (2.0.2)

Product Information:
Version: 2.0.2
Commit SHA-1 hash: a04b4bf512

Runtime Environment:
OS Name: Windows
OS Version: 10.0.15063
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\2.0.2\

Microsoft .NET Core Shared Framework Host

Version : 2.0.0
Build : e8b8861ac7faf042c87a5c2f9f2d04c98b69f28d
```

VS Code version: 1.18.1
C# Extension version: 1.13.1

## Steps to reproduce

In open project, right-click open folder. Click add folder to workspace, then select a different C# project.

## Expected behavior

Both folders will show intellisense, reference counters, have working goto definition, etc.

## Actual behavior

The extension only seems to work with the first folder/project in the workspace. All other folders/projects have no reference counter, cannot detect code errors, and when clicking goto definition, it prompts that no definition was found.