Add builds task to compile theme files

[damyanpetev]

Description

We should add a build task to compile the theme files (rather than having the output files in src).
This will also allow us to add autoprefixer processing as requested by @desig9stein

We could either compile the files in place (i.e. in the src folder exactly as they are now) or directly in dist. I'm leaning towards the first, just to minimize changes for now, but open to suggestions.

// CC @kdinev @mpavlinov @bazal4o @zdrawku

[IG-Stanimir-Dimitrov]

@damyanpetev , I would make an executive decision and suggest you add the build task and the Autoprefixer postprocessig to output the new theme structure files to the src folder.