#19 Implement (and fully document) a --merge-actions=""

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Implements (and fully documents) a --merge-actions=""
option to prcs, which overrides the default merge
action with the specified action (the argument of
the option is a string composed of characters
each of them specifying an action to try, if possible).
For example, using `--merge-action=ra' will replace
the working file with the file from PS (`r' action),
unless the working file does not exist and in this
case the file from PS will be added to PW (`a' action).

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