lang/org: major refactor

The motivation for this change was to rethink lang/org's flags. Many of
its former flags represented non-features. Therefore, its flags have
been reduced to five: +dragndrop, +ipython, +pandoc, +gnuplot and
+present. Everything else is included as org-load-hooks and treated as
reasonable defaults.

Other changes:
- Fixes #1502: don't autopair certain pairs when in a math region
- Fixes #1483: broken localleader in org-agenda
- Adds gnuplot support #1108
- Doom's org submodules have been moved into lang/org/contrib/, because
  I expect there will be *many* more to come, and I don't want to
  pollute the moudle's root.

init.example.el

@@ -125,13 +125,10 @@
        ;;nix               ; I hereby declare "nix geht mehr!"
        ;;ocaml             ; an objective camel
        (org              ; organize your plain life in plain text
-        +attach          ; custom attachment system
-        +babel           ; running code in org
-        +capture         ; org-capture in and outside of Emacs
-        +export          ; Exporting org to whatever you want
-        +habit           ; Keep track of your habits
-        +present         ; Emacs for presentations
-        +protocol)       ; Support for org-protocol:// links
+        +dragndrop       ; file drag & drop support
+        +ipython         ; ipython support for babel
+        +pandoc          ; pandoc integration into org's exporter
+        +present)        ; using Emacs for presentations
        ;;perl              ; write code no one else can comprehend
        ;;php               ; perl's insecure younger brother
        ;;plantuml          ; diagrams for confusing people more