Support highlighting fill-column overflows

This was previously semi-supported. There were two problems, however:

1. My fill-column value of 80 got out-of-sync with my white-line-column
   value of 100.
   Solution: globally define wpc/fill-column, set that to 80, and
   consume the new const in my config.

2. Nothing was activating whitespace mode.
   Solution: add whitespace-mode to prog-mode-hook.

This SO answer helped tremendously:
https://stackoverflow.com/questions/18855510/have-emacs-highlight-characters-over-80

emacs.d/wpc/packages/wpc-misc.el

@@ -70,7 +70,7 @@
       create-lockfiles nil)
 
 ;; ensure code wraps at 80 characters by default
-(setq-default fill-column 80)
+(setq-default fill-column wpc/fill-column)
 
 (put 'narrow-to-region 'disabled nil)
 

emacs.d/wpc/packages/wpc-ui.el

@@ -127,8 +127,11 @@
 ;; highlight lines that are over 100 characters long
 (use-package whitespace
   :config
-  (setq whitespace-line-column 100)
+  (setq whitespace-line-column wpc/fill-column)
-  (setq whitespace-style '(face lines-tail)))
+  (setq whitespace-style '(face lines-tail))
+  (add-hook 'prog-mode-hook #'whitespace-mode))
+
+
 
 ;; disable GUI scrollbars
 (when (display-graphic-p)

emacs.d/wpc/variables.el

@@ -18,5 +18,9 @@
     [mouse-5] [down-mouse-5] [drag-mouse-5] [double-mouse-5] [triple-mouse-5])
   "This variable stores all of the mouse-related keybindings that Emacs recognizes.")
 
+(defconst wpc/fill-column
+  80
+  "Variable used to set the defaults for wrapping, highlighting, etc.")
+
 (provide 'variables)
 ;;; variables.el ends here