Added header_style option.
Added header_style option.

Zizouz212/prettytable,bingkunyang/prettytable,pombredanne/prettytable,PGower/prettytable,nowox/prettytable,sha8e/prettytable,rs86/prettytable,NeilQ/prettytable,vsajip/prettytable,mihaelaapetroaie/prettytable,jackmaney/prettytable,criloz/prettytable,chakwing/prettytable,SantoshMathew/prettytable,memoryboxes/prettytable,codeOfRobin/prettytable,chaoallsome/prettytable,thomasantony/prettytable,kmonsoor/prettytable,dpedu/prettytable,chrinide/prettytable,cnbeining/prettytable,andrew-plunk/prettytable,arne-cl/prettytable,russgray/prettytable,tRexCheng/prettytable,alexkolar/prettytable,seckcoder/prettytable,fabiantheblind/prettytable,mseyne/prettytable,jpmieville/prettytable,jsionr/prettytable,3mty/prettytable,yunqianluo/prettytable