added support for recipeInstruction as string; added step formating

[JohannesFleischer]

This MR adds scraping support for websites which store the recipeInstructions as a string and not as an array of objects.

This includes chefkoch.de and therefore closes #278 and #498

---

Also included is a minor additional formatting of the Steps including paragraphs and numbering:

[iwontknow]

Is there someone who is able to check the merge request and merge it?
I cant add recipes because, i get this error everywhere i try to get a recipe.

[JohannesFleischer]

@iwontknow you could verify/test if my changes fix your problem or tell me which site you want to scrape from, and then I could take a look when I have some time.