Change how themes, fonts & modelines are loaded

WARNING: THIS IS A BREAKING CHANGE FOR THEME/FONT/NLINUM CUSTOMIZATIONS.

This change was motivated by the need to decouple theme and font loading
from the ui/doom module.

Now, it is doom-core's purview. Theme and fonts are loaded after
initfiles are read (attached to the doom-init-ui-hook hook), giving
other modules (especially private ones) a chance to change the theme or
fonts.

+ Refactor core-ui.el
+ New init hook: doom-init-ui-hook
+ Decouple theme/font loading from ui/doom
+ Load modelines are doom-init-ui-hook
+ New theme/font variables (replaces old ui/doom variables)
  + doom-theme
  + doom-font
  + doom-variable-pitch-font
  + doom-unicode-font
+ Change nlinum variables
  + doom-line-number-lpad
  + doom-line-number-rpad
  + doom-line-number-pad-char

Addresses #117

core/autoload/ui.el

@@ -59,3 +59,12 @@ window changes before then, the undo expires."
       (when (doom-quit-p "Close frame?")
         (delete-frame))
     (save-buffers-kill-emacs)))
+
+;;;###autoload
+(defun doom/reload-theme ()
+  "Reset the color theme currently in use."
+  (interactive)
+  (let ((theme (or (car-safe custom-enabled-themes) doom-theme)))
+    (when theme
+      (mapc #'disable-theme custom-enabled-themes))
+    (+doom|init-ui)))