Add color for Stan
Add color for Stan

Use the brick-red color of the [Stan logo](https://raw.githubusercontent.com/stan-dev/logos/master/logo.png).

stanhu/linguist,MostAwesomeDude/linguist,mwpastore/linguist,stanhu/linguist,c-lipka/linguist,JJ/linguist,erkyrath/linguist,sebgod/linguist,meunierfrederic/octocatlinguist,JJ/linguist,R4PaSs/linguist,meunierfrederic/octocatlinguist,mwpastore/linguist,ppaulweber/github-linguist,JJ/linguist,erkyrath/linguist,steventhanna/linguist,tschf/linguist,erkyrath/linguist,github/linguist,jtbandes/linguist,github/linguist,christop/linguist,stanhu/linguist,yyx990803/linguist,erkyrath/linguist,pchaigno/linguist,sebgod/linguist,jtbandes/linguist,R4PaSs/linguist,stanhu/linguist,yyx990803/linguist,pchaigno/linguist,meunierfrederic/octocatlinguist,osorgin/linguist,PsichiX/linguist,MostAwesomeDude/linguist,pchaigno/linguist,steventhanna/linguist,sebgod/linguist,pchaigno/linguist,ppaulweber/github-linguist,tschf/linguist,christop/linguist,github/linguist,yyx990803/linguist,c-lipka/linguist,github/linguist,MahmoudFayed/linguist,yyx990803/linguist,JJ/linguist,osorgin/linguist,MahmoudFayed/linguist,sebgod/linguist,PsichiX/linguist