Support multiple instances of the same plugin

[oprypin]

• Addresses blog plugin breaks when trying to use 2 blog instances squidfunk/mkdocs-material#4542

E.g.:

plugins:
  - blog:
      blog_dir: blog
  - blog:
      blog_dir: tutorial

Depending on what a plugin does, say if it "adds a thing" into the site, it can be reasonable to allow it to be specified multiple times in the plugins: list, so it can "add multiple things". Previously such a use case was completely not predicted in MkDocs, so it silently passes but is bugged - the plugin runs twice but with only one of the configs both times.

So, this PR addresses that by registering a plugin - foo: as 'foo', and then if another plugin - foo: appears, it gets registered as 'foo #2' (the name affects primarily just how it's referred to in warnings and errors).

By default, a warning will appear from MkDocs anyway, unless the plugin adds a class variable supports_multiple_instances = True.