Add subtitle under title to header

[ghost]

Hi, I added a subtitle (under title) to my jekyll minima blog. Wondering if it can be added to minima theme. Please let me know what you think. I will create a pull request.

Thank you!

[mattr-]

Do you have an example of what it looks like?

[ghost]

Here is the screenshot. I added a subtitle field in _config.yml.

Thank you!

[mattr-]

Could you open a PR for this? I'm not entirely sure that I'll add the feature yet but I would like to see your implementation.