org-e-html: Fix preamble and postamble

org-e-html: Fix preamble and postamble

* contrib/lisp/org-e-html.el (org-e-html--build-preamble): Renamed
  from `org-e-html-preamble'.  Properly call user-provided preamble
  function when appropriate.  Refactor code.
(org-e-html--build-postamble): Renamed from `org-e-html-postamble.
  Properly call user-provided preamble function when appropriate.
  Refactor code.
(org-e-html-template): Small refactoring.
(org-e-html--translate): Use :html entries in `org-export-dictionary'
instead of :utf-8.
(org-e-html--build-meta-info): Renamed from `org-e-html-meta-info'.
Add docstring.
(org-e-html--build-style): Renamed from `org-e-html-style'. Add
docstring.
(org-e-html--build-mathjax-config): Renamed from
`org-e-html-mathjax-config'.  Add docstring.