Config overrides on the command line

We have too many daemon commandline flags and the ones we have often duplicate config options. Instead of having both, we should add a flag for temporarily setting config options:

```
ipfs daemon --with Gateway.Offline true \
--with Addresses.Gateway /ip4/127.0.0.1/tcp/8080
```

This is significantly more verbose than the current system but is hierarchical and easier to maintain.