Add grouping support for Month + Year

[JamoCA]

I locally modified "widget-grouping.js" to add support for grouping by month & year (as opposed to only year, month, day, week or time values.)

I added this class to the table header column:

group-date-monthyear

and added this translation to the date function in the file widget-grouping.js.

part === 'monthyear' ?  wo.group_months[time.getMonth()] + ' ' + time.getFullYear() :

Could this be added to the master?

[Mottie]

Hi @JamoCA!

Nice idea! I'll include this in the next update.