Support prelude/start-process

If you refer to the previous commit where I change shell-command usages to
start-process function calls, you'll see the rationale for why I prefer
start-process.

This commit introduces a more ergonomic API for start-process that fits most of
my current use-cases of it. This cleans up the code. I have introduced a bug in
the way that I'm tokenizing the COMMAND value. I've tracked that with a
TODO. For now it only affects the `xmodmap -e '<command-string>'` calls, which
isn't too disruptive.

configs/shared/.emacs.d/wpc/string.el

@@ -22,11 +22,14 @@
 (defconst string/test? t
   "When t, run the tests.")
 
-;; Strings
 (defun string/hookify (x)
   "Append \"-hook\" to X."
   (s-append "-hook" x))
 
+(defun string/split (y x)
+  "Map string X into a list of strings that were separated by Y."
+  (s-split y x))
+
 (defun string/ensure-hookified (x)
   "Ensure that X has \"-hook\" appended to it."
   (if (s-ends-with? "-hook" x)