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/editor/evil/+commands.el

@@ -4,6 +4,7 @@
 ;;; Custom commands
 ;; Editing
 (evil-ex-define-cmd "@"            #'+evil:macro-on-all-lines)   ; TODO Test me
+(evil-ex-define-cmd "R[ead]"       #'+evil:read)
 (evil-ex-define-cmd "al[ign]"      #'+evil:align)
 (evil-ex-define-cmd "ral[ign]"     #'+evil:align-right)
 (evil-ex-define-cmd "enhtml"       #'+web:encode-html-entities)