move files to support fisher plugin standards (#293)

functions/fish_greeting.fish

@@ -0,0 +1,10 @@
+function fish_greeting -d "What's up, fish?"
+    set_color $fish_color_autosuggestion
+    uname -nmsr
+
+    # TODO: `command -q -s` only works on fish 2.5+, so hold off on that for now
+    command -s uptime >/dev/null
+    and command uptime
+
+    set_color normal
+end