Run multiple jobs after each other

Hi,

Thanks for such a great project!

I would love to always run the tests no matter if clippy or check finds any errors. I believe that in https://github.com/Canop/bacon/issues/42 they talk about something similar (always seeing the output of the test job).

Motivation: For me it is not uncommon to be working on something but still have some clippy/check issues (usually unused functions, especially when writing tests). It would be nice if I could instruct bacon to always run the tests even though I get some warnings from the other checks.

What I tried: I tried chaining the jobs together using the `on_success = "job:clippy"` syntax on my `test` definition but then when I make a change the tests are not re-run as bacon is now on the clippy job. As I might have a warning in clippy I cant add a `on_success = "back"` as it is not seen as a success. I did try setting `allow_warnings = true` and then back on success but it did not seem to work.

Being able to run both clippy/check and tests and show the output of both (or just the tests pass! header) always would be a nice feature.

Here is an example of what I have tried.

```toml
[jobs.clippy]
command = [
"cargo", "clippy",
"--all-targets",
"--color", "always",
]
need_stdout = false
allow_warnings = true
on_success = "back"

[jobs.test]
command = [
"cargo", "test", "--color", "always",
"--", "--color", "always", # see https://github.com/Canop/bacon/issues/124
]
need_stdout = true
on_success = "job:clippy"
```

Start by reading the job chaining behavior described in this issue and the related issue #42, then inspect the handling of `on_success`, `allow_warnings`, and the `back` job transition. Done means a configuration can run clippy/check and tests after the same change, while retaining the relevant output even when an earlier job reports warnings or errors.

Written by the indexing model from the issue text.