Fixes the extension issue for -t syntax highlighting, and the filename for stdin


Its a cheap fix, man, but it does work :)

I tested all these permutations:

```
cat blah | gist -t rb
```

```
gist -t sh blah.rb
```

```
rvm info | gist -t sh
```

Of course if no extension is given, or if there is none on the file, the gist isn't syntax highlighted on GH.
As I said, cheap and dirty but its working.