rename 'theme' to 'layout' in all instances except `highlight_theme`

[Liopia]

This pull request resolves #29 as described

[xy2z]

Thanks @Liopia

Maybe I should have been more clear but Code/syntax highlighting theme should not be changed to layout..

• README.md line 12: should still be 'themes'.
• README.md line 57+59 should be 'theme' instead of 'layout'
• config/config-example.yaml line 27+28: should be 'themes' instead of 'layouts'.
• src/templates/main.html line 17 should not be changed.

Other than that, it's great

[xy2z]

Thanks!