Add laborHours field

[ruphy]

Add the following optional field to the main section:

laborHours: [integer] An estimate of total labor hours spent by your organization/component across all versions of this release. This includes labor performed by federal employees and contractors. See the Code.gov summary for calculating labor hours.

cc @RicardoAReyes

[ruphy]

see: https://github.com/GSA/code-gov/blob/master/LABOR_HOUR_CALC.md