Reserving ranges

[raulk]

Add the ability to reserve ranges in the multicodec table, à la Protobuf.

[Stebalien]

We can "soft" reserve small ranges in the 2 byte range (16,256) codes, we can probably hard reserve small ranges in the 3 and 4 byte ranges (2e6 and 2.5e8 codes respectively). Assuming there's a good argument for it.

By "soft" reserve, I mean "try to leave this range open unless we need to use it for something else".

[ntninja]

Could we designate a private-use area in the 4-byte range? Specifically, I hereby nominate this range to be 0x300000 - 0x3fffff (just above the designated media type area of #159) and will use this until/unless some other area is decided upon.

[rvagg]

^ @ntninja open a PR to note that in the README and check for their absence in the validation script and we can probably lock that in.

[ntninja]

@rvagg: Done!