add processor_chain arg to stdlib.ProcessorFormatter

[Akuli]

Fixes #334

• Added tests for changed code.
• Updated documentation for changed code.
  • New functions/classes have to be added to docs/api.rst by hand.
  • Changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives. Find the appropriate next version in our __init__.py file.
• Documentation in .rst files is written using semantic newlines.
• Changes (and possible deprecations) are documented in the changelog.

I'll add tests if any of this is actually a good idea. I'm new to structlog and there might already be a way to do what I wanted to do in #334 .

[hynek]

I'm sorry, but we can't just remove such an important parameter.

Shouldn't it be possible to write a processor that wraps a chain? Processors should be composable, so maybe we should add more helpers to achieve that.

[Akuli]

That makes sense. This just isn't a good approach.

[hynek]

Hi @Akuli, ironically I'm working on something very similar. Would #365 work for you?

[Akuli]

Thanks! Seems like it would work, although I no longer work on the project where I wanted this.