Edit files inplace like sed -i

[bobpaul]

Issue 23 from the original repository asks for sed -i like behavior. Something like

$ echo -e "123\n457" > file
$ sed -i 's/7/6/' file
$ cat  file
123
456

This seems like a good addition, but working with files are currently rather second class in pyp. I think this would take some re-thought. Maybe if pyp runs without any STDIN and the -t option, it treats the file input as pp instead of fpp.