Fix: overwriting footer.html breaks math

[jpvg10]

Yes, you can customize the footer with param footerContent in config.toml. But another way of doing it is creating your own footer.html like so:

my-site/
├── layouts/
|   └── partials/
|   |   └── footer.html  <--
├── themes/
|   └── typo/

However if you do that, it breaks math on the site, because the "math block" is inside the theme's footer.html.

@tomfran this is just one way I could think of solving the issue, let me know what you think :)

P.s. Thanks for the amazing theme!