sfeed_update: add filter(), order() support per feed + improvements

Pass the name parameter to the functions and add these to the pipeline. They
can be overridden in the config.

- add the ability to change the merge logic per feed.
- add the ability to filter lines and fields per feed.
- add the ability to order lines differently per feed.
- add filter example to README.

- code-style:
  - fetchfeed consistency in parameter order.
  - change [ x"" = x"" ] to [ "" = "" ]. Simplify some if statements.
  - wrap long line in fetchfeed().
  - use signal names for trap.

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