Added support for custom head content

[Arafo]

Just as simple as that, add support to include a custom-head.html file so you can add your own head content

[jeffreytse]

@Arafo Thanks for the contribution, could you explain the advantage why we need another custom-head.html file rather than to change the head.html directly?

[Arafo]

It's an easy way to add custom head code without changing head.html.

One potential use case, you installed the theme through Gemfile and now you want to add something new to the head section. Without a custom head file you have to add _layouts/head.html to your project overriding the file from this repo with the same content plus what you want to add.

That's a bit inconvenient as head.html content could change in the future if someone adds a new plugin to this theme and so, you have to update your head file in your own project.

Adding a way to include custom head content avoids this issue.

[jeffreytse]

@Arafo Thanks for the explaining. Hope you have a good day! : )