Added custom header and footer the same way as in BeautifulHugo.

[excitoon]

Hi.

I would appreciate if you keep underscores in file names in order to have more or less standard names for custom files.

Kind regards,
Vladimir.

[excitoon]

Hi, @christianmendoza

Can you please take a look at my patch?

Kind regards,
Vladimir.

[escalate]

Hello @excitoon

thank you for your PR. I received the ownership for this project and will try to include your code into my fork as quick as possible.

For the moment I don't understand the need of this change. It would be very helpful if you could explain some use cases for this change.

The code looks like that somebody could break the whole layout with the help of footer_custom or header_custom if it used not properly. It is not very safe to use it in my opinion.

Another point is that an implicit documentation inside the files is not very helpful.
It would be better to write all posible configuration options inside the README.md and explain how to and under wich circumstances to use them.

If you like you could create a new PR for the repository at https://github.com/escalate/hugo-split-theme.