Generate file name if needed.

[slabruyere]

Here is my suggestion to tackle issue #48 .

I'm doing just like GitHub for its gists:

• if there is no note name, generate one randomly note:{32 hexadecimal characters}
• if there is no file name notefile1 or gistfile1 and so on...

[lauthieb]

Thanks a lot @slabruyere for your contribution!