Added line specification
Added line specification

Required by Libre and Open office, and correct by spec. When we get a
requirement to customize the line width/shape this should be extracted
out into a Line class.

clarkcutler/axlsx,romuloceccon/axlsx,woto/axlsx,raghavendra-prithvi/axlsx,alexanderfrankel/axlsx,legitscript/axlsx,kappiah/axlsx,VueOps/axlsx,sr-education/axlsx,mromulus/axlsx,Debiancc/axlsx,sh-ft/axlsx,pedrogarrett/axlsx,randym/axlsx,bradstewart/axlsx