Rewrite CLI to use mixins for logging purposes

[ZauberNerd]

At the moment the CLI does not use a mixin and log output for build and
server is done in the hops-express and hops-webpack package.

Inspired by the CLI package in untool, we now use a CLI mixin which
takes care of logging for express and webpack.

TODO: With this PR the webpack readme will be removed - we should
make sure that the necessary information is retained someplace else.

Edit: The express and webpack readmes are now part of: #655

• All commit messages adhere to the appropriate format in order to trigger a correct release
• All code is written in untranspiled ECMAScript (ES 2017) and is formatted using prettier
• Necessary unit tests are added in order to ensure correct behavior
• Documentation has been added

[dmbch]

LGTM!

[jhiode]

LGTM 👍

[jhiode]

We could add extends Mixin here to make it clear(er) it's a mixin.

That would also be good in order to not break as soon as it is actually required to inherit from Mixin.
And thats what we are doing anywhere else.

[ZauberNerd]

I don't think we can - at least not while the mixin is still part of @untool/core, because then the CLI (which might be installed globally) will have a dependency to @untool/core.

But what we could do, is to make the Mixin a separate package in untool and then we could add it (or rather hops-mixin) as a dependency to the CLI package.

/cc @dmbch

[dmbch]

As the Mixin base class is just a five-liner, I am opposed to releasing it separately. I personally think that for this very use-case, it is completely fine to have a standalone class.

[jhiode]

LGTM