Add archetype per language

[bep]

Working with #4535 I noticed an issue with archetypes with multiple languages with each its own content directory. I will improve that "what site/language does this content belong to" logic, but to make it non-ambiguous, I will also add a (optional) --lang flag:

hugo new --lang=jp post/my-post.md

Doing that, it would also be good if each language could have its own set of archetype templates.

Since #4535 introduces a directory-based approach, we should probably prefix these to make them non-ambigous. My suggestion:

archetypes
├── _languages
│   └── jp
│       └── default.md
├── default.md
└── my-bundle
    ├── featured.png
    └── index.md

/cc @RickCogley @kaushalmodi @regisphilibert @spf13 @anthonyfok and gang.

Note that I plan to wrap this up fairly quickly and include this in a "Revival of The Archetypes #2" release on Monday. There is also the new slice related template funcs that, with Go 1.11, I think we should get out there sooner rather than later. So Monday it is.

[regisphilibert]

Do we need the lang flag? Can't the language be guessed by filename/location?

Do we need an archetype _language directory? This prefixed directory introduces something rather unusual in Hugo. I wonder if we could do without. Couldn't we use the same file system as content files?

archetypes
├── default.md
└── default.jp.md
└── my-bundle
    ├── featured.png
    ├── index.md
    └── index.jp.md

[bep]

@regisphilibert I think you need to look at #4535 and #5233 and #5234 in full.

Do we need the lang flag? Can't the language be guessed by filename/location?

How would your examples look with a content directory per language? (which I think is a more common setup for sites that really need something like this).

[bep]

OK, thinking a little about this, this isn't as obvious as I first thought. I think I will get #4535 working properly and see if I get some clarity from that.