Allow user to enable rainbow-identifiers-mode without making it default

Per syl20bnr's comment on PR 6192, the package should always be
installed with the colors layer, but only hooked into prog-mode when the
variable is active. Renamed the variable
colors-enable-rainbow-identifiers-by-default to clarify this, and
modified the documentation.

layers/+themes/colors/README.org

@@ -28,12 +28,12 @@ file.
 
 ** Enable rainbow-identifiers
 
-To enable the package =rainbow-identifiers= set the variable
-=colors-enable-rainbow-identifiers= to =t=:
+To enable =rainbow-identifiers-mode= by default, set the variable
+=colors-enable-rainbow-identifiers-by-default= to =t=:
 
 #+BEGIN_SRC emacs-lisp
   (setq-default dotspacemacs-configuration-layers '(
-    (colors :variables colors-enable-rainbow-identifiers t)))
+    (colors :variables colors-enable-rainbow-identifiers-by-default t)))
 #+END_SRC
 
 Saturation and lightness of identifiers can be set per theme by adding

layers/+themes/colors/config.el

@@ -11,7 +11,7 @@
 
 ;; Variables
 
-(defvar colors-enable-rainbow-identifiers nil
+(defvar colors-enable-rainbow-identifiers-by-default nil
   "If non nil the `rainbow-identifers' package is enabled.")
 
 (defvar colors-enable-nyan-cat-progress-bar nil

layers/+themes/colors/packages.el

@@ -43,7 +43,6 @@
 
 (defun colors/init-rainbow-identifiers ()
   (use-package rainbow-identifiers
-    :if colors-enable-rainbow-identifiers
     :commands rainbow-identifiers-mode
     :init
     (progn
@@ -62,7 +61,8 @@
         :documentation "Colorize identifiers globally."
         :evil-leader "tCi")
 
-      (add-hook 'prog-mode-hook 'rainbow-identifiers-mode)
+      (when colors-enable-rainbow-identifiers-by-default
+        (add-hook 'prog-mode-hook 'rainbow-identifiers-mode))
 
       (defun colors//tweak-theme-colors (theme)
         "Tweak color themes by adjusting rainbow-identifiers."