Fish config refactor

Move things into sub-files and sub-directories
Add ensure_path function to make sure stuff doesn't get spammed multiple
times into the PATH

fish/os/os.fish

@@ -0,0 +1,9 @@
+# OS-specific configuration
+switch (uname)
+  case Linux
+    source ~/.config/fish/os/linux.fish
+  case Darwin
+    source ~/.config/fish/os/osx.fish
+  case '*'
+    echo 'Unknown OS. No specifc config'
+end