Replaced hardcoded call to vi with $EDITOR to allow the user to use their favourite editing tool.

Test case:

```bash
misc/vyl (master) $ find . -name README.md 
./README.md
misc/vyl (master) $ EDITOR=vi; ./vyl 
# Loads the README.md file in vi
misc/vyl (master) $ find . -name README.md 
./README.md
misc/vyl (master) $ EDITOR='subl -w'; ./vyl 
# Loads the README.md file in Sublime Text 2
```