Allow single-language pages

[ranbena]

This is a PR to allow single-language pages.

The problem:
My site's intl is defined with 2 languages but a some pages are only in one language (for instance a page for a conference offered for English speakers only, therefore irrelevant to translate to additional locales).

But there is no way to indicate to the plugin that a certain page should "opt out" of all language page generation.

The solution:
Defining a page in a directory with the language name is easy and clear.
Here's an example:

// languages - en + es
|-- /src
    |-- /pages
        |-- foo.jsx      <-- normal: generates /foo, /en/foo, /es/foo
        |-- a/bar.jsx    <-- normal: generates /a/bar, /a/en/bar, a/es/bar
        |-- en/baz.jsx   <-- single language: generates en/baz only 🔥

Related to #152.

[ranbena]

Notice, I'm already using this in production on a website of mine called citytree.

This is my pages folder.
You can see practical-ecology has both English and Hebrew.
While abracadabra is only in English (Hebrew yields 404 page).