Added initial search functionality

When you press `/` in normal mode, you enter the new `SearchMode`, which simply allows you to enter your search term, and then stores that term in `Editor.LastSearchCommand`.

Then, when you press `n` or `N` in normal mode, it will use the `Search` command from `commands/search.go`.

Let me know if you have any feedback / thoughts on this. I think it works pretty well, and that it is open enough for other commands to make use of it, like you mentioned in #20 

Thanks!