feat: add support for plain C
feat: add support for plain C

Add support for plain C files. C++ was allowed, but C was not.

jrvidal/docco,whitetrefoil/docco,DoomHammer/docco,jalleyne/docco,lobrien/docco,MaxwellRebo/docco,osharim/docco,gvithellyna/docco,oponder/docco,lbn/docco,guigrpa/docco,osharim/docco,actano/docco,oponder/docco,robertmackenzie/docco,tbremer/docco,mhkeller/docco,MaxwellRebo/docco,guigrpa/docco-extended,whitetrefoil/docco,lobrien/docco,mhkeller/docco,ShawnHuang/docco-multidir,robertmackenzie/docco,DoomHammer/docco,jrvidal/docco,gvithellyna/docco,ShawnHuang/docco-multidir,Hellyna/docco,lbn/docco,guigrpa/docco-extended,jalleyne/docco,guigrpa/docco,Hellyna/docco,tbremer/docco