Add clipcopy() and clippaste() generic cross-platform CLI clipboard functions.

Change copydir, copyfile, and coffee plugins to use them, instead of the Mac-only `pbcopy` command.

plugins/copyfile/copyfile.plugin.zsh

@@ -1,5 +1,7 @@
+# Copies the contents of a given file to the system or X Windows clipboard
+#
+# copyfile <file>
 function copyfile {
-  [[ "$#" != 1 ]] && return 1
-  local file_to_copy=$1
-  cat $file_to_copy | pbcopy
+  emulate -L zsh
+  clipcopy $1
 }