Inline hooks

Here is a proposal to have inline hooks with a build job. So currently there a few places you can have hooks:
1. Global hooks
2. Per-pipeline hooks
3. Plugin hooks

How about we have inline hooks as well, like:

```
steps:
- command: "my-script.sh"
hooks:
environment: "echo 'run at the every beginning'"
post-checkout: "echo 'after the code is checked out'"
post-artifact: "echo 'run after artifacts are uploaded"
```

The list of hooks we could support here:
- environment
- pre-checkout
- checkout
- post-checkoutt
- pre-command
- ~~command~~
- post-command
- pre-artifact
- post-artifact

What do you think?