Support camelCase for test method names

[Jeehut]

Currently, when I follow the Swift guidelines and write test names like this:

    func testBasicRendering() {
        expect(snapshotView).toMatchSnapshot()
    }

I get files named like this:

reference_testbasicrendering@3x.png

I would have expected the name to be reference_testBasicRendering or at least reference_test_basic_rendering, but Snap doesn't seem to recognize camelCased method names which are the Swift default convention. This should be definitely added IMHO.