Introduce nvm function for fish

This is a wrapper around nvm. It's main function is to check and promote environment variables changes from nvm to fish.

A little more is done to setup the environment in case that hasn't been done yet:

* install a stable node version if it doesn't exist
* set the default to stable alias if that hasn't been done yet

Due to those actions, the first execution of `nvm` might take some time.

nvm.fish

@@ -0,0 +1,24 @@
+function nvm --description "Node version manager" -a nvm_command nvm_command_arg1
+  set nvm_dir ~/.nvm
+  set nvm_ $nvm_dir/nvm
+
+  # This sets some environment vars that we need to set too
+  if test "$nvm_command" = "use"
+    eval $nvm_ use --print-paths $nvm_command_arg1 | sed -re "s|^(\w+=)|set -x \1|g" -e "s|[=:]| |g" | grep "set -x"  | .
+  else
+    # Make sure we can use node
+    if test ! (which node)
+      # Have we installed node at all ?
+      if eval $nvm_ ls | grep "N/A" > /dev/null
+        echo "No node installation found, installing stable"
+        eval $nvm_ install stable
+      end
+      # Make sure we have a default picked
+      if eval $nvm_ ls default | grep "N/A"
+        eval $nvm_ alias default stable
+      end
+      nvm use default
+    end
+    eval $nvm_ $argv
+  end
+end