Support explicit whitelist of exposed APIs

[lidel]

Originated from ipfs/ipfs-companion#478

Summary

There are contexts in which we don't want to expose sensitive APIs such as name.publish or config.

TODO

• For now we can throw an error in pre hook provided to createProxyServer , but ideally only whitelisted APIs should be exposed via window.ipfs.
• (optional, nice to have) We should add a method window.ipfs.availableApis which just exposes a list of available function names.