Issure 73 edit function handle certain cases

if todo is `FLOATING`
    edit [id] -start -end must be given as "required" parameter to change to `TIMED`.
else todo is `DEADLINE`
    edit [id] -start will just add the startdate and changed to `TIMED`.
else todo is `TIMED`
    edit [id] -start will just update the startdate.