Improved chowdown integration (better import, new export)

[buergi]

I extended the chowdown import integration to serve me during the import of my recipe database.

• Added waiting_time
• Added url
• Fixed handling of trailing description (after ---), was parsed to descriptions variable but never imported to recipe
• Improved handling of missing attributes (e.g. empty title raised error)
• Length limit for keywords
• Import long description as "Notes" step
• Detection for header lines in ingredients
• For recipes with ingredients only (no directions) add dummy step

Additionally I also added an export integration for the chowdown format.