Running SCENARIOs: Omit "SCENARIO" in selection

Say I have some `SCENARIO` defined like so:

``` C++
SCENARIO("foo", "[foo][bar][baz]") {
// ...
}
```

I argue that the intuitive interface for selecting to run only the tests for this specific scenario is: `./test_executable foo`.

However, given that the `SCENARIO` binding prepends its descriptor with `Scenario:`, we're required to instead select for the specific scenario like so: `./test_executable "Scenario: foo"`, which seems less than ideal.

The issue concerns SCENARIO descriptor binding and command-line selection through ./test_executable. Start by tracing where the Scenario: prefix is added and how selection arguments are matched. Done means ./test_executable foo selects that scenario without requiring the Scenario: prefix, while existing selection behavior remains intact.

Written by the indexing model from the issue text.