Stack test specific subset

### Command

``` bash
stack test --fast --test-arguments '-m "<identifier>"'
```

The `<identifier>` can be pretty much anything:

- Part of / a module's name
- A `describe` / `it` description

#### Example

Say you have the following `Spec`:

``` haskell
module MyApp.Foo.BarSpec (spec) where

spec :: Spec
spec = parallel $ do
  describe "baz" $ do
    it "qux" 
```

- To run all the unit tests under `Foo`, run:

``` bash
stack test --fast --test-arguments '-m "Foo"'
```

- To run all the unit tests in the `BarSpec` module, run:

``` bash
stack test --fast --test-arguments '-m "Bar"'
```

Note it's just `Bar`, not `BarSpec`.

- To run the subset of unit tests under `baz`, run:

``` bash
stack test --fast --test-arguments '-m "baz"'
```

- To run a specific single unit test, run:

``` bash
stack test --fast --test-arguments '-m "qux"'
```