feature request: VimCalc on current buffer.

I'd like to be able to have a history of the calculations that doesn't goes away when closing VimCalc window. It would be even better to have a separated history for each task/project.

I think this could be accomplished through a command that activate VimCalc on the current buffer, instead of on a new one. Through vim filetypes it would be possible to have VimCalc activated automatically when opening such history files.

Thanks for sharing this awesome plugin!