jtbm37

Roslyn has support for it [here](https://github.com/OmniSharp/omnisharp-roslyn/blob/2c6c8a481fbdd05c4c06f5ddc720d3a9315d6062/src/OmniSharp.Abstractions/Models/v1/SignatureHelp.cs) and has [tests](https://github.com/OmniSharp/omnisharp-roslyn/blob/2c6c8a481fbdd05c4c06f5ddc720d3a9315d6062/tests/OmniSharp.Roslyn.CSharp.Tests/SignatureHelpFacts.cs). We just need to show the signature when typing `(`. Optionally user can call the function when point is inside parentheses.