Add command to locate files

[iridakos]

Users should be able to execute something like:

stup locate @ 2020-04-27
# or
stup locate today
# or
stup locate today -c "meetings"

and retrieve a list of the filenames mainted by stup for the given day (for a specific or for all categories):

<category>    <path-to-file-for-the-specific-date>
<category>    <path-to-file-for-the-specific-date>