Add :Read ex command & 'SPC i p' keybind

This can be used to extract paths from evil-ex style paths. e.g. the
following inserts the stdout into the current buffer (assuming we're in
~/some/project/filename.c):

  :R!echo %:P      ~/some/project
  :R!echo %:t      filename.c
  :R!echo %:e      c
  :R!echo %:r      filename
  :R!echo ~/another/project/%:t:r.h
    ~/another/project/filename.h
  :R %             contents of current file

http://vimdoc.sourceforge.net/htmldoc/cmdline.html#filename-modifiers
has a full list of vim filename modifiers. Doom doesn't support all of
them, but it does support most of them.

modules/config/default/+evil-bindings.el

@@ -468,10 +468,11 @@
 
       ;;; <leader> i --- insert
       (:prefix-map ("i" . "insert")
-        :desc "From clipboard"                "y"   #'+default/yank-pop
+        :desc "Evil ex path"                  "p"   (λ! (evil-ex "R!echo "))
         :desc "From evil register"            "r"   #'evil-ex-registers
         :desc "Snippet"                       "s"   #'yas-insert-snippet
-        :desc "Unicode"                       "u"   #'unicode-chars-list-chars)
+        :desc "Unicode"                       "u"   #'unicode-chars-list-chars
+        :desc "From clipboard"                "y"   #'+default/yank-pop)
 
       ;;; <leader> n --- notes
       (:prefix-map ("n" . "notes")