Handle spaces in config options

[jakkdl]

There's a bunch of different ways of handling spaces in comma_separated_list to fix --trio200-blocking-calls. Pending on what happens with PyCQA/flake8#1771

• handle ['a', '->', 'b'] as well as ['a->b'] with fancy logic
• don't use comma_separated_list and write a custom action (more attractive if option_manager.add_option ignores action when reading from config PyCQA/flake8#1770 is fixed)
• Require users to wrap values in quotes or something. Though some quick testing looks like this doesn't help.
• Wait for it to be fixed upstream

[jakkdl]

Since 1771 was closed as invalid, this should likely be ignored for --startable-in-context-manager (since spaces aren't valid identifiers anyway), but --trio200-blocking-calls will need a custom type that handles this.

[Zac-HD]

Closed by #93. (you need a keyword per issue #, annoyingly)