Add "pymdownx" and "pyyaml" in dependencies

[dimastbk]

Hi!

Please add pymdownx and pyyaml in dependencies. dependencies.json should look like thi:

{
    "*": {
        ">=3119": [
            "markupsafe",
            "mdpopups",
            "pygments",
            "python-jinja2",
            "python-markdown",
            "pymdownx",
            "pyyaml"
        ]
    }
}

Without this I see in console ImportError: No module named 'yaml' and mdpopups not work.

See: https://github.com/facelessuser/sublime-markdown-popups/blob/master/docs/src/markdown/api.md