WIP: initial implementation of Edit-Content command

[rkeithhill]

This is a work in progress but I wanted to show what I've got so far. There are plenty of design decisions to be made on this command. Right now it only edits files. Should it have an InputObject parameter? There are optimizations for loading smaller files completely into memory vs using a temp file for larger files, etc. Also, I wrote the original code ~6 years ago so there may be better ways in .NET now to do things.

The main thing I need to do now is to test this code.

Fix #19