Add fromRaw function to loop

[tversteeg]

I needed to add this function to be able to use a luv loop in my code.

auto loop = uvw::Loop::fromRaw(luv_loop(...));
loop->run<uvw::Loop::Mode::NOWAIT>();

[skypjack]

Wouldn't it be better something like this?

    static std::shared_ptr<Loop> create(uv_loop_t *ext) {
        auto ptr = std::unique_ptr<uv_loop_t, Deleter>{ext, [](uv_loop_t *){}};
        return std::shared_ptr<Loop>{new Loop{std::move(ptr)}};
    }

[tversteeg]

I was considering this as well, but seeing that using the create() constructor would be the default recommended way and you wanting to discourage people to use raw C pointers I thought it would be better to create a different constructor for it.

[coveralls]

Coverage remained the same at 97.624% when pulling 8f9aed9 on tversteeg:master into ea4b6c8 on skypjack:master.