Add an option to force vim plug update (#795)

* Add an option to force vim plug update (fix #751)

* Rustfmt

* Update src/config.rs

Co-authored-by: M*C*O <mcofficer@gmx.de>

Co-authored-by: M*C*O <mcofficer@gmx.de>

src/steps/upgrade.vim

@@ -11,7 +11,11 @@ endif
 if exists(":PlugUpgrade")
     echo "Plug"
     PlugUpgrade
-    PlugUpdate
+    if $TOPGRADE_FORCE_PLUGUPDATE
+        PlugUpdate!
+    else
+        PlugUpdate
+    endif
 endif
 
 if exists(":PackerUpdate")