changed file_exists to file to allow urls

[ToshY]

Fixes #55

[char0n]

Thanks for contributing! Please have a look at my code review comments. Also if we're introducing this feature, it would be good to add tests for it. Please use this testing file for testing: https://github.com/char0n/ffmpeg-php/blob/master/tests/data/test.mp4?raw=true. I can imagine following tests:

• testing if FFMpegProvider can handle URL
• testing if FFProbeProvider can handle URL
• testing if Movie can handle URL
• testing what happens when provided URL cannot be fetched

[ToshY]

Well the CI seems to be fine with it, but I honestly don't know if it's okay like this, as I normally don't write tests. So I just tried to mimic the other tests and ended up with this.
Let me know if there's something that needs to be changed.

[char0n]

The tests satisfy the requirements. Nicely done. I'll just do some minor cleanup in them before the release.