Param completion

Attempt at #419.

Ctrl-Shift-Space now works, and you don't have to be positioned right after the '('.  

When deciding whether to pop up a Parameter completion, Instead of checking what character we're at, check what character was typed. This allows Ctrl-Shift-Space to work at these locations:

```fsharp
let date = System.DateTime.Now.ToString(§  // Auto-opens here
let date = System.DateTime.Now.ToString(§)
let padded = date.PadRight(§   // Auto-opens here
let padded = date.PadRight(§2)
let padded = date.PadRight(2§)
```

Try it out and lets discuss here.