Categorize time spent
### Description
Being able to tag time spent by area (was it dev, research, investigating, fix, whatever?) would help give more insight.
### Proposal
- Add a `~label` argument to `/spend` command or something to that effect.
- Output should create a note saying something to the effect of:
`Loic Nageleisen @lnageleisen added 30m of time spent on ~research at 2018-01-25 about 19 hours ago`
### Links / references
issue