New features

• Column layouts that let you structure slides into columns.
• Support for percent margin rather than only a fixed number of columns.
• Spacebar now moves the presentation into the next slide.
• Add support for center footer when using the template mode.
• Breaking: themes now only use colors in hex format.

Fixes

• Allow using sh as language for code block (#3).
• Minimum size for code blocks is now prioritized over minimum margin.
• Overflowing lines in lists will now correctly be padded to align all text under the same starting column.
• Running cargo run will now rebuild the tool if any of the built-in themes changed.
• alignment was removed from certain elements (like list) as it didn't really make sense.
• default.alignment is now no longer supported and by default we use left alignment. Use default.margin to specify the margins to use.