BREAKING CHANGE: previously base was not preserved and all CIDs would
be normalised to base58btc when asking for their string representation.

The default will change to base32 in https://github.com/multiformats/js-cid/pull/73/files

The idea behind this change is that we shouldnt lose information when
the user passes us a base encoded string, but keep it and use it as
the default base so toString returns the same string they provided.

I'd like this as a fix for ipld explorer, which currently forces all
CIDs into base58btc, seee: ipfs/ipfs-webui#999

License: MIT
Signed-off-by: Oli Evans <[email protected]>

Adds tests for
- base other than 'base32btc' for CIDv0
- codec other than 'dag-pb' for CIDv0

License: MIT
Signed-off-by: Oli Evans <[email protected]>

store the string form of the CID as a non-enumerable property where
an instance is constructed from a valid string, or after the first
call to toBaseEncodedString where the supplied base matches the
default base.

adds tests for string caching, and some missing test cases.

License: MIT
Signed-off-by: Oli Evans <[email protected]>

Add flow type definitions which would improve workflow for dependent libraries that happen to use flow type-checker.

feat: add flow typedefs

- Clarify how to use the mutlibaseName param in the README.
- Update the usage section with clear examples