Allows custom external links via conf.py

[Zulko]

Very simple change allowing users to define as many external links as they want from conf.py

In conf.py:

html_context = {
        "external_links": [
          ("Github", "https://github.com/username/repo"),
          ("Other", "https://github.com/username/repo")
        ]
    }

Result (top right corner):

[schettino72]

Nice.

@Zulko , sorry I am not being very responsive. No time to work in this project lately.

I just gave you commit access to the project. Feel free to merge your improvements directly into master.

[Zulko]

Thanks very much, I'll try to keep things clean. Since you gave me commit access, the next changes to come are:

• A new CSS spreadsheet specifically for formatting sphinx-generated autodocs elements (those aren't formatted at all at the moment and that's doesn't look nice)
• Allow users to use their own custom CSS spreadsheet.

[schettino72]

btw. this is an item of #15 .

I guess Sphinx recommends using html_theme_options to define theme related configuration instead of html_context.

[Zulko]

OK, I'll look into it.