feat: enabling pubsub and ipns-pubsub via config flags

[jwh]

Depends on ipfs/go-ipfs-config#145

[aschmahmann]

@jwh it looks like CI isn't running. Is it possible you have a CircleCI account that is following your fork rather than ipfs/go-ipfs?

From https://circleci.com/docs/2.0/oss/#build-pull-requests-from-forked-repositories

Note: If a user submits a pull request to your repository from a fork, but no pipeline is triggered, then the user most likely is following a project fork on their personal account rather than the project itself of CircleCi, causing the jobs to trigger under the user’s personal account and not the organization account. To resolve this issue, have the user unfollow their fork of the project on CircleCI and instead follow the source project. This will trigger their jobs to run under the organization when they submit pull requests.

[lidel]

Added docs, and sharness tests to ensure we check both ways of enabling pubsub.
Let's wait for CI. If there are still issues, I'll open this as a new PR. sharness is green

[lidel]

Note to self:

• sharness tests both ways of enabling pubsub (cli+config), both pass
• sharness tests both ways of enabling ipns-over-pubsub (cli+config), both pass
• needs to be switched to go-ipfs-config release with feat: pubsub and ipns pubsub flags go-ipfs-config#145 before this is merged