Add support for generating lvm-index.7 and lvm-categories.7 man page

This adds support for -i|--index and -a|--categories to man-generator. The generator will then accept a list of files (from man subdirectory) to process and generate the index/categories man page based on that. This makes it easier for users to see all the man pages installed and navigate through them, either indexed alphabetically or in categorized view.