refactor: config.ipfsHash is now optional

[UnKnoWn-Consortium]

ipfsHash supplied via hls.config will be prepended to the filename with an appended slash only when available. FIlename can now be a CID path (e.g. QmYjtig7VJQ6XsnUjqqJvj7QaMcCAwtrgNdahSiFofrE7o/path/to/file). It is backward compatible. Close #23.