Implement defaultHashAlg property

[vmx]

After merging #27 all formats need to implement a defaultHashAlg property (see the spec for more information).

See the discussion at ipld/js-ipld#82

Instead of opening an issue on every repository, just let people know on this issue that you're working on it and then link to the PR.

• https://github.com/ipld/js-ipld-bitcoin
• https://github.com/ipld/js-ipld-dag-cbor
• https://github.com/ipld/js-ipld-dag-pb
• https://github.com/ipld/js-ipld-ethereum
• https://github.com/ipld/js-ipld-git
• https://github.com/ipld/js-ipld-raw
• https://github.com/ipld/js-ipld-zcash

[richardschneider]

All changes have been pushed to master!