feat: teach pinnacle#decorate() to accept a comma-separated list

feat: teach pinnacle#decorate() to accept a comma-separated list

Incidentally, first time I've tried running docvim on this machine, and
I couldn't get any of the old build or install methods working, due to
bit rot:

    git clone ...
    cd docvim
    brew install stack # didn't have stack on this machine
    stack build # build fails
    stack install docvim # install fails
    cabal install docvim # install fails
    cabal v2-build # victory!

After that I could do the actual doc-gen with:

    ./dist-newstyle/build/x86_64-osx/ghc-8.4.4/docvim-0.3.2.1/x/docvim/build/docvim/docvim \
      -c ~/code/pinnacle \
      -v \
      ~/code/pinnacle/doc/pinnacle.txt \
      ~/code/pinnacle/README.md