IPNS pinning: ipfs name follow

cc @vyzo

```
USAGE
ipfs name follow - Track changes to IPNS names.

SYNOPSIS
ipfs name follow [--pin=true] [--allow-expired=true] [--]

ARGUMENTS

- IPNS path of the name that should be followed.

OPTIONS

--pin bool - Always pin the name's most recent content. Default: true
--allow-expired bool - Allow lookup even if a name's TTL is reached. Default: true

DESCRIPTION

The 'ipfs name follow' command can be used to track changes to an IPNS name.
A followed name will be refreshed in the background in intervals, so that
lookups through an 'ipfs' command or the HTTP API are always instant and up-to-date.

When called with '--pin' (default), after refreshes the name's new content
is pinned. This is equivalent to 'ipfs pin replace /ipfs/QmOld /ipfs/QmNew'.

When called with '--allow-expired' (default), lookups of the name through
the 'ipfs' command or the HTTP API always return the last known content,
even if the TTL has been reached and the name has expired. If this is the
case, a warning is printed along with whatever is the command's respones.

The combination of '--pin' and '--allow-expired' allows for convenient
offline use cases. For example, on a long flight I'd like to have IPFS docs
with me, and I'd like them to be usable at /ipns/docs.ipfs.io as always.

Open question: pubsub

Open question: refresh after ttl/2 or whatever
- /ipns/QmFoo: ttl = ipns-ttl
- /ipns/dnslink.net: ttl = min(dns-ttl, ipns-ttl)

Open question: storage of followings
```