Load zsh customizations from custom/zsh/before/* and custom/zsh/after/* instead of filename globbing [Close #79]

* File pattern globbing was too brittle. If you had plugins incorrectly
   named, or had only a before or after directory, things would break in
   nonobvious ways.

custom/zsh.sample/before/before.sample.zsh

@@ -0,0 +1,16 @@
+#
+# Samples of what you can do in *.before.zsh files.
+# You can create as many files as you like, or put everything in one.
+#
+
+# append your own plugins. the $plugins at the end includes the plugins
+# defined by YADR.
+plugins=(osx ruby vagrant $plugins)
+
+# ignore plugins defined by YADR and use your own list. Notice there is no
+# $plugins at the end.
+plugins=(osx ruby vagrant)
+
+# set your theme.
+export ZSH_THEME="kennethreitz"
+