Handle multiline options

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).