feat(init) display peer id in init command

use case:

Just configured and installed a node.

benefits:

1) reduces friction when setting up a new node

2) reveals useful details about the workings of the system. It's the
user's first encounter with her node's identity. The tour can build on
the user's knowledge.

```
ipfs (maybebtc-november) λ. ipfs init -f
initializing ipfs node at /Users/btc/.go-ipfs
generating key pair
peer identity: QmcRbn41Vc2CvbpLYfN36mAWusErKWvAAHbq92LPra2gFT
```