Add script to cat all input files.

Why I want this:

1) to be able to easily compile all the input in one go to test out a new markdown compiler
2) as a test suite for vim-markdown: https://github.com/plasticboy/vim-markdown

Tests there cannot be easily automated, so we need one big file to manually check things out.

If this is accepted I'll do my best to convince vim-markdown people to adopt this repo as their test suite. I have push rights there.

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Renamed `REAMDE.md` to `README.markdown` to not conflict with the input `.md` files.