Handle multiline options

[DimitriPapadopoulos]

Clean up options expecting lists before using them, as they may contain newlines.

Examples:

• Enclosing command-line arguments in quotes may introduce newlines in option values:

  $ codespell -S "A, B,
  >               C, D, E"

• INI files may contain multiline values:

  [codespell]
  skip = A, B,
       C, D, E,

In all the above cases, the option parsing mechanism keeps the newlines (and spaces) . We need to clean up, by splitting on commas and stripping each resulting item from newlines (and spaces).

Fixes #3399.

Very much related to #2767. Both pull requests introduce an option to flatten the options list after splitting on commas, and both need tests!

Supersedes #3299.

[DimitriPapadopoulos]

@larsoner Would you agree to merge this PR that fixes a codespell bug? Full tests already added.

[larsoner]

@DimitriPapadopoulos I simplified the logic by avoiding the Optional stuff and just always having a List[str] and making use of stuff like any([]) == False, feel free to merge if you're happy with the change