enhancements for offline use

[jkall]

Additions:

1. an option in the config file to set file suffix for locally saved text files (by default .txt but user may want to use .md or .mkdn instead)
2. a feature (File-menu and Ctrl+Shift+M) to open the locally saved text file, for the selected note, in default application
3. simplenote module is only needed if user wants to sync with simplenote (and not for those only working offline)

[yuuki0xff]

@jkall Is your request enhancement to notes_as_txt feature?