A special character for horizontal and vertical centering text on a slide

[visit1985]

This may be a good idea to avoid endless formating sessions.

[SikhNerd]

Several formatters use -> Text to be centered <- which I am fairly partial to. I think this would be a necessary feature.

[orclev]

That provides horizontal centering but it would be nice to support vertical centering as well, unless you dictate that horizontally centered content must also be vertically centered. Admittedly trying to mix vertically centered content with non-vertically centered content could cause layout problems, so it might be best to just abandon the concept of vertically centering anything, or else put a limitation that when using centering no other non-centered content can appear on that slide.

[visit1985]

@orclev I think I'll not implement vertical centering, as it conflicts with automatic line wrapping (#15). Content is printed line by line, and we don't know the actual line count until all lines are printed. So we can not easily calculate how many lines should be skipped at the top.