feat: add flag for reading specified puzzle part

[tyrone-wu]

Add an optional --part flag that accepts values 1 or 2 in the read subcommand for displaying/reading the specified part of the puzzle. If the flag is not specified, this defaults to displaying the entire puzzle.

This also adds some display consistencies. When output includes part 1 puzzle, the --- Part One --- subheader is included in the output. Additionally, if only displaying part 2 puzzle, the --- Day Foo: Bar --- header is included in the output.

Fixes: #27