[TW-1760] Allow duration fields to be formatted

_Marco Villegas on 2016-01-25T01:37:16Z says:_

It would be great to have a way to output the raw value of a duration field, for instance to manipulate the values from a script.

Based on date formatters, duration fields by default print the human friendly version of a time lapse, that is a great default value, but a formatter like estimate.raw could output the raw value stored in seconds in the data files.

For instance an alias like:
alias.daydurations=ls due:today rc.report.ls.columns=estimate.raw rc.report.ls.labels=Est rc.verbose=nothing

could output
{code}
3600
1800
{code}

instead of 1h and 30m