org-export: Add `org-export-read-attribute' for normalized attr lines

* contrib/lisp/org-export.el (org-export-read-attribute): New function.
* testing/lisp/test-org-export.el: Add test.
org-export: Fix template for derived back-ends

* contrib/lisp/org-export.el (org-export-as): Fix template for derived
  back-ends.
org-element: Fix item interpreter when item doesn't start with a paragraph

* contrib/lisp/org-element.el (org-element-item-interpreter): Fix item
  interpreter when item doesn't start with a paragraph.
* testing/lisp/test-org-element.el: Add test.
Description list: improve regexp consistency

* lisp/org-list.el (org-at-item-description-p,
org-list-item-body-column): Make the inline regexp more consistent
with `org-list-full-item-re', the inline regexp "Description list
items" from `org-set-font-lock-defaults and others'.

This resolves some issues with M-q (org-fill-paragraph) on description
lists.
org-export: Fix code comments
org-e-latex: Set Babel language according to LANGUAGE keyword

* contrib/lisp/org-e-latex.el (org-e-latex-babel-language-alist): New
  variable.
(org-e-latex--guess-babel-language): New function.
(org-e-latex-template): Set babel language according to LANGUAGE keyword.
org-e-latex: Babel language set through LANGUAGE keyword is loaded last

* contrib/lisp/org-e-latex.el (org-e-latex--guess-babel-language):
  Make sure Babel language set through LANGUAGE keyword is loaded
  last.
org-e-odt.el: Use `org-export-read-attribute'
org-export: Follow EXPORT_TITLE property when exporting subtree

* contrib/lisp/org-export.el (org-export-get-subtree-options): Make
  sure point is at an headline and buffer isn't narrowed before
  looking for EXPORT_TITLE property.
* testing/lisp/test-org-export.el: Add test.
org-e-latex: Follow `not-in-toc' specification for tags

* contrib/lisp/org-e-latex.el (org-e-latex-headline): Follow
  `not-in-toc' specification for tags.
org-e-latex: Make headlines more robust in toc

* contrib/lisp/org-e-latex.el (org-e-latex-headline): Allow curly
  brackets and replace square brackets with parenthesis in table of
  contents when tags are removed.
org-export: Fix error when calling "l" from dispatcher

* contrib/lisp/org-export.el (org-export-dispatch): Fix error when
  calling "l" from dispatcher.
org-element: Allow footnote references in an item tag

* contrib/lisp/org-element.el (org-element-object-restrictions): Allow
  footnote references in an item tag.
org-e-latex: Support for footnote references in an item tag
org-e-latex: Refactor code

* contrib/lisp/org-e-latex.el (org-e-latex--delayed-footnotes-definitions):
  New function.
(org-e-latex-footnote-reference, org-e-latex-item): Use new function.
org-e-latex: Allow previously definied footnotes in item tag

* contrib/lisp/org-e-latex.el (org-e-latex-footnote-reference): Allow
  previously definied footnotes in item tag.
* lisp/ob-R.el: Handle quoted string values to device header arguments
* lisp/ob-R.el: Removed reduntant list of header arguments
Revert "* lisp/ob-R.el: Removed reduntant list of header arguments"

This reverts commit 829af89f48fd13e6efefbe4e22a7c147f9d264ba.
org-element: Parse contents of DATE keyword

* contrib/lisp/org-element.el (org-element-parsed-keywords): Parse
  contents of DATE keyword.