Runs loader option

Created by: rettigl

I address some of the issues raised with adding the runs options as general option to the loader interface in a separate PR, to be merged with flash-new first. I overhuled the general implementation in the base-loader and processor, and reworked your implementation in the flash loader. Some things are yet preliminary. In particularly, I am not sure if you really need several source folders where to look for your runs. Your doc strings and typing information is very inconsistent there. Also, if runs should be int or string, a list or just one entry is rather unclear. I expect it a sequence of strings for now... We can explode it to all kinds of Unions if you want, but let's be clear and consistent.