fix: allow jumping back to column in column layout

[mfontanini]

Before this change you could create a column layout, jump to column X, then Y, then go back to X and it worked but it would start over at the very top row of column X. This meant it would overwrite content you wrote in that column the last time you entered it.

This changes that behavior so it behaves as you'd expect: re-entering a column will cause the cursor to continue at the last row you modified in that column. This, in combination with pauses, allows you to create presentations where content shows up in multiple columns progressively, e.g. pause, show content on column 0, show content on column 1, pause, show more content, etc.

As an example, the following presentation:

# Text before

<!-- column_layout: [1, 1] -->

<!-- column: 0 -->
# hi

<!-- column: 1 -->
hi

<!-- column: 0 -->
bye mom

this is text

<!-- column: 1 -->
hi

hi

hi

hi

<!-- reset_layout -->

# Text after

Renders like this:

Fixes #346