Use expand-file-name instead of concat
Rename compilable asset files
Use other file extensions for compilable assets
Implement (jekel--publish-sass)
Implement (jekel--publish-coffee)
Make (jekyll--render-layout) a macro again

Without a macro the body is evaluated to early.
Define macro in (jekel--define-markup-helpers)

Te rename macros we have to use (macrolet) otherwise the body of the
helper macros gets evaluated too early.