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

Issue #84 suggested the ability to edit a new file. I've now added this.

At the same time I've added some pre-amble that is prefixed to files when editing (and stripped again when it is encrypted and saved). This is to help those who are less familiar with the edit mode or do not use it often.

There is a new option --no-preamble that turns this off, which can be placed in the configuration file if a user never wants to see this.