File saving

[rr-]

...yeah.

• :w to save changes in place
• :w /path/to/file to save changes to a file
• Prompt warn and bail out if target file in :w /path/to/file is to be overwritten
• Prompt warn and bail out if the currently edited file has no name (e.g. it's a new file and user tries to :w)
• :w! and :w! /path/to/file
• Prompt before exiting when the user hasn't saved changes