Add support for double height/double width

[rbanffy]

VT-100 compatible terminals support line-based attributes double-height (DECDHL) and double-width (DECDWL). It'd be useful to support those for headers, perhaps with a command-line switch (since terminal support is spotty and doesn't work at all under tmux/screen).

On xterm:

With Apple's Terminal.App:

[vinayak-mehta]

Thanks for suggesting this feature! I didn't know about this.

I'll have to see how well these attributes work on other OSes/emulators, and also have a way for present to not fail on unsupported systems.