Allow users to include additional Markdown pages

[milmazz]

This PR allows the user to include additional Markdown pages, via command line you can use as many --extra (or the alias -e) as you want:

$ bin/elixir ../ex_doc/bin/ex_doc "EEx" "1.1.0-beta" "lib/eex/ebin" -m "EEx" -u "https://github.com/elixir-lang/elixir" --source-ref "64bb812f15e8b9ec078f76cfd8e26578bf2ddaa0"  -o doc/eex -p http://elixir-lang.org/docs.html \
--readme README.md -e CHANGELOG.md -e CONTRIBUTING.md

$ ls doc/eex/{README,CHANGELOG,CONTRIBUTING}.html         
doc/eex/CHANGELOG.html doc/eex/CONTRIBUTING.html doc/eex/README.html

For Mix there is a new options called extras:, example:

# file: mix.exs
docs: [source_ref: "v#{@version}", main: "Ecto",
            source_url: "https://github.com/elixir-lang/ecto",
            readme: "README.md", extras: ["CHANGELOG.md"]]]

Closes #316

/cc @josevalim @dignifiedquire

[dignifiedquire]

Love it, even styling wise I think this is good for now!

[josevalim]

We can deprecate it now. We are before 1.0. :)

[josevalim]

Actually, just remove it.

[josevalim]

AWesome work @milmazz! I have added two comments, feel free to remove the README option, and let's ship it!

[lpil]

Yay!