Implement a fitsort mode for `fitsheader`.
Implement a fitsort mode for `fitsheader`.

bsipocz/astropy,saimn/astropy,MSeifert04/astropy,dhomeier/astropy,StuartLittlefair/astropy,mhvk/astropy,stargaser/astropy,lpsinger/astropy,saimn/astropy,stargaser/astropy,aleksandr-bakanov/astropy,StuartLittlefair/astropy,pllim/astropy,mhvk/astropy,pllim/astropy,StuartLittlefair/astropy,astropy/astropy,aleksandr-bakanov/astropy,astropy/astropy,pllim/astropy,aleksandr-bakanov/astropy,lpsinger/astropy,dhomeier/astropy,bsipocz/astropy,stargaser/astropy,bsipocz/astropy,StuartLittlefair/astropy,astropy/astropy,astropy/astropy,saimn/astropy,stargaser/astropy,StuartLittlefair/astropy,MSeifert04/astropy,dhomeier/astropy,dhomeier/astropy,larrybradley/astropy,lpsinger/astropy,larrybradley/astropy,lpsinger/astropy,larrybradley/astropy,MSeifert04/astropy,larrybradley/astropy,saimn/astropy,bsipocz/astropy,dhomeier/astropy,mhvk/astropy,pllim/astropy,mhvk/astropy,larrybradley/astropy,astropy/astropy,saimn/astropy,aleksandr-bakanov/astropy,MSeifert04/astropy,pllim/astropy,lpsinger/astropy,mhvk/astropy