Add vim modeline magic

[Rogdham]

Automagically enforce a 2 space indentation to vim users.

Nothing fancy, just one comment line at the end of the file. The alternative is to put this line at the very top of the file, just after the shebang (i.e. on line number 2). Please tell me if you prefer the alternative more.

[progrium]

Do you know how many other popular editors use this vim data?

[Rogdham]

I'm sorry but I have quite no idea (I only know about a plugin for gedit…) .

I can't say that I have a strong opinion on this PR. I find it pretty convenient as a vim user, but at the same time I understand that having editor-specific lines in your source code is not that great.