Move vim flag cast to outside of get (#1486)

With this config:

```toml
[tool.black]
line-length = 79
```

In neovim, this is loaded as a string which later causes an exception to
be thrown. This makes sure the value is always cast to an int

plugin/black.vim

@@ -190,7 +190,7 @@ def get_configs():
     toml_config = {}
 
   return {
-    flag.var_name: toml_config.get(flag.name, flag.cast(vim.eval(flag.vim_rc_name)))
+    flag.var_name: flag.cast(toml_config.get(flag.name, vim.eval(flag.vim_rc_name)))
     for flag in FLAGS
   }