Add !*, +* and -* operators

I suggest new operators which allow to compose Includes and Excludes a bit more flexibly. I'm not sure it's OK. Maybe these operators should combine two instances of FileIncludes instead of a FileIncludes and a list of file paths (strings). However, I cannot see how we may combine them.

The suggested operators is useful for scenarios like the following: 

```fsharp
let x86UnitTests =
    [ testDll "x86UnitTests"
      testDll "x86UnitTests2" ]

let x86ETETests =
    [ testDll "x86ETETests"
      testDll "x86ETETests2" 
      testDll "x86ETETests2" ]

let x86Tests = x86UnitTests @ x86ETETests

let all = !+ (src @@ (@"*Tests\bin\" + config + @"\*Tests.dll"))

Target "x64 tests" <| fun _ ->
    all
    -* x86Tests
    |> Scan
    |> NUnitParallel id

Target "x86 unit tests" <| fun _ ->
    !* x86UnitTests
    |> Scan
    |> NUnit id

Target "x86 ETE tests" <| fun _ ->  
    !* x86ETETests
    |> Scan
    |> NUnit id
```