support extra options for find(1)?

[greg-minshall]

hi. in my environment, where i have .org files scattered all over the place, including in ${HOME}, i would like to search for org files in a specified set of directories, but not in any of those directories' subdirectories (i.e., not in the trees rooted at the directories in org-grep-directories). for find(1), this could be done by passing the -maxdepth 1.

i've implemented something like this here (and, also tried to deal with issue #6), with an option org-grep-find-options. i'd be happy to turn into a git patch.