Add support for storing notes in markdown text files

[gamato]

In "Settings -> Notes -> Storage" one can choose:

• Single Database
• Plain text files
• Rich text files
• HTML documents

Please add a new option to store notes in Markdown text files.

Thus nvALT could be easily used in cooperation with other note taking apps that appeared recently, especially on other devices where nvALT is not available. (Simplenote is way too simple) Thanks!

[goblindegook]

Markdown files are plain text, so you can pick the second option and then configure md as the default file extension below.