We have no Default suffix, so we will not assume twig is the default. An...

... app path is required or ./templates will not render correctly.

There is no defaulting to *.twig if we do not include a template suffix, it will default to empty 
string and try to render 

home/hello 
home/index

not

home/hello.twig
home/index.twig

Without using an application path, there is no logic for SlimController to locate its includable files suchas vendor/autloader, the templates directory, etcetra