Add extension support

[dignifiedquire]

• Browser tests
• Extension support (needed for https://github.com/ipfs/js-ipfs-ipld)

Depends on:

• Setup dignified.js #4
• Add configurable extension support js-ipfs-repo#36

cc @diasdavid

[daviddias]

Can we make the 'extension' be transparent and have a type instead, as for 'ipld', and 'protobuf', from UX perspective, it will be easier to read that we want to read the block that is stored as protobuf or as ipld, rather than having to know the extension (which .data doesn't really mean anything).

[daviddias]

master nicpicker :P

[dignifiedquire]

Not sure I agree with that. If we go this route we have to encode the mapping of type to extension into the Block and need to change it everytime we might want to add a new type. If we just use extension we can pass any new thing through without ever having to touch the internals of Block again.

[daviddias]

I understand, hopefully, we won't to change again though :). It does make an js-ipfs contributor have to have domain knowledge of how blocks are put in disk.

[daviddias]

nice way to do it :)