Add 7z container support

[rolandwalker]

relevant Casks must use depends_on_formula unar

Tests are handled similarly to the (updated) PR #1992 -- they will be skipped unless the developer has unar installed.

The system /usr/bin/file cannot recognize 7z files, so extension and magic_number methods were added to criteria.rb to aid in determining filetypes.

Unlike #1992, this is implemented in terms of an abstract base class Cask::Container::UnarBase. We can use the base class to support Stuffit containers and whatever else is desired from the broad number of filetypes supported by unar. (I plan only 7z, Stuffit, and RAR).

Since unar provides a consistent interface across many archive types, it would actually be possible to skip having a Cask::Container::SevenZip class entirely in favor of a Cask::Container::Unarable which would come last in the list of container classes and function as a kind of catch-all. However, for both technical and stylistic reasons, I think being explicit is a better approach.

[nanoxd]

If this allows less complexity in the project, I'm all for it.

[muescha]

Can this depends_on_formula unar better goes from the cask to the unarable lib?

[rolandwalker]

rebased

[rolandwalker]

@nanoxd , if you meant that the "catch-all" notion would reduce complexity, not really. Once the unar base class here is introduced, then, for example, adding Stuffit containers takes only 8 lines of code. And the problems of the "catch-all" class are many: it's literally requesting undefined behavior. I can imagine something like #2654 being complicated by the fact that unar also reads DMG files -- the file could "fall through" and our code could take two different cracks at it.

@muescha if you are curious about how the design decisions were reached, you may refer to #1992 and #2305 .

[muescha]

my comment i continued in #3207