Add a way to programmatically interact with timers

`katja_vmstats:get_timer/1`, `katja_vmstats:start_timer/2` and
`katja_vmstats:stop_timer/1` can be used to programmatically interact
with timers.
Add internal stop_collection_intervals/1
Document timer functions in README
Comment out EUnit coverdata import
Update erl_opts
Bump Katja dependency
Bump version