Support default accumulative break times

[sanak]

Currently, break XX mins overwrite existence break time, but always accumulating past values (in a head) is a bit difficult, and I like declarative break times than accumulated ones, so it may be better to support default accumulative break times.

Here is my idea.

# Initial break
break 10 mins
> @foo I changed break time to 10 mins
# Accumulate break times
break 20 mins
> @foo I changed break time to 30 mins

# Overwrite break times by "set" keyword
break 120 mins set
> @foo I changed break time to 120 mins

[iosefa]

Also related to #25