Add roles and versions as new dimensions (in addition to language)

[bep]

The implementation here is still early and non-working. So, the role part of this has been thought about a lot (at least by me, e.g. in #519). The versions part of this is a little vaguer, but I think it helps with the end result to add them both in one go.

So,

---
title: Hugo Rocks!
roles: [public]
versions: [v1.2.3, `v2.*`]

Note that the Glob pattern support in v2.* is also new – but I suspect we will need something like that for this.

As for languages, it will be possible to configure a role/version as the default (e.g. the "current version"). I guess it also makes sense to control whether the default should be rendered to a sub folder or not.

As to the order of the prefixes, role needs to come first, I guess.

• I guess we will also add versions and roles to the file mount config.
• I'm thinking we could also add some kind of "counter identifier" in content filenames, e.g. content/mybundle/index.§1.md and content/mybundle/index.§2.md to allow having multiple variants of the same content path.

New API

• Site.Version; a struct with Name, Site ...
• site.Versions; a slice of Version. Ordered by Weight (if set) ascending, else semver order, newest first.
• Site.Role; a struct with Name, Site ...
• site.Roles; a slice of Role.

Note that we currently have Site.Sites, which is, with the above setup coming, a little confusing, so we should possibly also consider to add Site to `Language.

New Config

I've thought about this, and decided to not make up any new clever syntax, but just mimic how we do it for languages.

defaultContentRole = "guest"
defaultContentRoleInSubdir = true
defaultContentVersion = "v2.0.0"
defaultContentVersionInSubdir = true

[roles]
[languages."guest"]
disabled = false
weight = 43
[versions]
[versions."v2.0.0"]
disabled = false
weight = 42