Auto merge of #241 - vyaslav:master, r=fmoko

feat(watch): show hint while watching

`rustlings hint ...` command is not convenient when doing exercises with `rustlings watch`.
This PR makes it possible for user to type `hint` while running `watch`  and get hint text for exercise which is currently failing.
e.g.
```rust
...
  --> exercises/variables/variables1.rs:13:36
   |
13 |     println!("x has the value {}", x);
   |                                    ^ not found in this scope

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0425`.

type 'hint' to get help:
hint
Hint: The declaration on line 12 is missing a keyword that is needed in Rust
to create a new variable binding.
```

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