Specify generated method call name

[a-voronov]

Give ability to specify generated method's call name.

One of the reasons - is to avoid names conflict.
Example below will generate two calls with similar name fCall using current implementation.

i.e.

// sourcery: Mock
protocol P {
    // sourcery: mock = fab
    func f(a: A, b: B)
    // sourcery: mock = fxyz
    func f(x: X, y: Y, z: Z)
}