Support ctags

This allows Atom editor to quickly navigate to predicate definitions and
supports autocomplete.
Very primitive 'clk ls' command

This just shows the tail end of the most recent timeline file.  The next
step is to parse that file to create marks then display marks in a nice
human-readable (or machine readable format).
Factor out most_recent_file/1

The logic for most_recent_file/1 is not quite right yet, but it's being
used in a few different places.  Having a single predicate will make it
easier to fix this code later.