Bolt hooks

While using bolt for our monorepo we discovered one problem, which can be just named "bolt hooks".

In short - running `bolt` will lead to
- `bolt` running [yarn install](https://github.com/boltpkg/bolt/blob/master/src/commands/install.js#L57)
- `yarn` running all hooks, like `prepare` or `postinstall`
- `bolt` linking the package
- ... nothing more ...

## Proposal
- [ ] add `bolt:postinstall` hook. It would be enough for the beginning
- [ ] do something with yarn hooks running before bolt. Probably moving `link` to `pre-postinstall` hook or something.