Add metadata to shims

When a shim is created, add plugin metadata so we can later know which shims belong to which plugins, this will help aid with removing unused shims on uninstall. See #67

```
# asdf-plugin: ${plugin_name}”
```

Thanks to @duijf for the metadata proposal.

lib/commands/reshim.sh

@@ -48,9 +48,11 @@ write_shim_script() {
   if [ -f $plugin_shims_path/$executable_name ]; then
     cp $plugin_shims_path/$executable_name $shim_path
   else
-    echo """#!/usr/bin/env bash
+    cat <<EOF > $shim_path
+#!/usr/bin/env bash
+# asdf-plugin: ${plugin_name}
 exec $(asdf_dir)/bin/private/asdf-exec ${plugin_name} ${executable_path} \"\$@\"
-""" > $shim_path
+EOF
   fi
 
   chmod +x $shim_path

test/fixtures/dummy_plugin/bin/install

@@ -3,3 +3,10 @@
 mkdir -p $ASDF_INSTALL_PATH
 env > $ASDF_INSTALL_PATH/env
 echo $ASDF_INSTALL_VERSION > $ASDF_INSTALL_PATH/version
+
+# create the dummy executable
+mkdir -p $ASDF_INSTALL_PATH/bin
+cat <<EOF > $ASDF_INSTALL_PATH/bin/dummy
+exec echo dummy
+EOF
+chmod 755 $ASDF_INSTALL_PATH/bin/dummy

test/install_command.bats

@@ -42,3 +42,11 @@ teardown() {
   [ "$status" -eq 0 ]
   [ $(cat $ASDF_DIR/installs/dummy/1.2/version) = "1.2" ]
 }
+
+@test "install_command should create a shim with metadada" {
+  run install_command dummy 1.0
+  [ "$status" -eq 0 ]
+  [ -f $ASDF_DIR/installs/dummy/1.0/env ]
+  run grep "# asdf-plugin: dummy" $ASDF_DIR/shims/dummy
+  [ "$status" -eq 0 ]
+}