Add preamble and non-existant file handling
Change comment prefix and rephrase preamble
Respond to comments made by @gtmtech

 - Remove commented line
 - Make adding prefix easier to read
 - Put file reading in rescue block
Alter test to modify and check encrypted file
Merge pull request #90 from sihil/sihil-edit-command-changes

Edit mode: edit new file and add pre-amble help text