Redirect input, output and error at demand

I start this feature based on [this conversation](http://lists.tiker.net/pipermail/pudb/2013-May/000174.html). 

I need to debug a python [script which acts like a filter](https://github.com/albfan/filter-word-diff), so I'm interested on provide input and see output while debugging

I don't want to mess my code with std redirections when debugger can be told to do so

to use:

```
$ pudb -i /some/file -o /dev/pts/1 script.py
```
The defaults should be `None`, meaning don't change anything. `/dev/stdin` doesn't exist on Windows.