Allow restricting tag generation to files listed by custom commands This adds a new setting, g:gutentags_file_list_command, which specifies command(s) to use to list files for which tags should be generated, instead of recursively examining all files within the project root. This is useful in projects using source control to restrict tag generation to only files tracked in the repository. This setting is conceptually similar to CtrlP's ctrlp_user_command option. This implements the feature requested in https://github.com/ludovicchabant/vim-gutentags/issues/90

syntax:regexp
(^|/)tags$