Update setupfile, better handling of LaTeX headers

All `html_head_extra's are replaced with `html_head's since they have
the same effect.

LaTeX headers are now handled in a separate file for easier handling of
the LaTeX code.

content/headers.tex

@@ -0,0 +1,14 @@
+% LaTeX headers
+\usepackage[total={17cm,24cm}]{geometry}
+\usepackage{xltxtra}
+\setmainfont{Charis SIL}
+\usepackage{indentfirst}
+\usepackage{enumitem}
+\usepackage{hyperref}
+\usepackage{xcolor}
+\hypersetup{colorlinks=true,linkbordercolor=red,linkcolor=blue,pdfborderstyle={/S/U/W 1}}
+\usepackage{newunicodechar}
+\newunicodechar{’}{'}
+\newunicodechar{…}{\ldots}
+\newunicodechar{ }{~}
+\newunicodechar{ }{~}