Add feature: "add css links" #138

[epsilonhalbe]

• add a new flag -c/--cssincludes such that multiple css files can be added to your standard html via elm-reactor -c "http://localhost:8000/static/mycss" -c "https://example.com/css/example.css"

this is my first endeavour into elm land (though i am a long-time haskell user).

I created a new flag for elm-reactor to include (multiple) css files in the generated html when I view the app to some elm-file.

hope this fixes issue #138 - if there is something missing please tell me.

[process-bot]

Thanks for the pull request! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

[epsilonhalbe]

is there anything lacking? - I would be glad to add it if there is sthg missing

[epsilonhalbe]

Is this project dead??