Implement Outputs

[hoshsadiq]

As discussed in #50, this is a replacement for #51.
I didn't really change much so in terms of the layout it's pretty much the same. I pretty much just rebased, and made a few small changes.

To address a few concerns:

• In terms of layout, there's not a lot to add. The only thing I could think of was adding a blue header bar to the "outputs for module root", but it didn't seem appropriate.
• It's hard to not store values as strings, mostly because we do still need strings. So storing data as something other than strings, will be a lot of work.
• Module outputs do not seem to be available unlike pre-0.12. I think the original code in Add support for outputs (Fix #50) #51 does deal with it, but I've not tested.

Happy to make any additional changes you like.

Fixes #50, closes #51

[raphink]

Linting is failing in Travis CI.

Other than that, let's go with this implementation 👍

[coveralls]

Coverage decreased (-0.1%) to 13.618% when pulling 846c632 on hoshsadiq:outputs into 92eb7ba on camptocamp:master.

[hoshsadiq]

Thanks, didn't realise for some reason.

On a side note, following message on travis-ci.org:

Please be aware travis-ci.org will be shutting down in several weeks, with all accounts migrating to travis-ci.com. Please stay tuned here for more information.

Might be worth migrating to something else (note travis-ci.com severely limits build minutes for free usage).

[raphink]

Yes, I'm aware of the Travis CI situation. I need to find time to migrate our projects to GitHub Actions. We have hundreds of repos to migrate though, and only so much time to do it 😁