Rework list options to make them less confusing

[kevina]

How Aspell handles list arguments is non-standard and confusing to many users. Consider fixing this to be more inline with standard behavior.

• rename list options to use singular nouns rather than plural, provide (hidden) alias for the plural version to avoid breaking existing usage
• if a non list option is specified more than once make sure that options later in the command line override previous values (this should be the case but it needs to be verified)
• if a list option is specified more than once then combine the two options into one list

None of these changes should break anything. The --add-OPT, --reset-OPT, and --rem-OPT will still be supported. The behavior of --OPT will change to be closer to --add-OPT but for comparability more than one value can still be listed.