`ipfs config` and live daemon configuration state

#### Version information:

```
go-ipfs version: 0.4.11-
Repo version: 6
System version: amd64/darwin
Golang version: go1.9
```

#### Type:

Bug

#### Severity:

Low

#### Description:

`ipfs config` interacts with the configuration file on disk, but not with the current configuration state of the daemon. This means that the operator cannot know for sure which configuration is currently applied, and which requires a daemon restart.

Below are a few possible ways to fix the issue.

1. Add an optional flag `ipfs config [--live=false]`. On `--live=true`, all config commands operate on the live config state of the running daemon, instead of the config file.

2. Add a new command `ipfs config reload`, to reload the live configuration from disk without requiring a daemon restart.

3. Add a new command `ipfs config diff`, to show the differences between the live configuration and the on-disk configuration. Optionally, this could be implemented as `ipfs config reload --dry-run`.