Optional file type argument to Autoformat command

A recurring situation for me is to come across badly formatted code on the internet that I'd like to just quickly beautify to take a look at. Since I have Vim open I paste it in a new buffer.

Currently I need to:

```
:set ft=json
:Autoformat
```

This pull request merges the two into one step:

```
:Autoformat json
```

Thereby saving a whole 8 key strokes. =)