Add support for variable last column widths in Markdown tables

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Add support for variable last column widths in Markdown tables.

Often the last column in a Markdown table is the widest, and adding new rows with long pieces of text can cause the whole table to need reformatting, which isn't ideal in a Git diff view.

If the last column in a Markdown table can be variable width, the example given in https://github.com/bitwiser73/MarkdownTableFormatter/issues/22 is a single-line change, rather than a seven-line change.

Closes: https://github.com/bitwiser73/MarkdownTableFormatter/issues/22.

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@bitwiser73 What do you think of this PR? I'm using the changes locally and they work great for me.

I think the code could be optimized so that rather than making the change after the initial column widths are calculated, the final column setting is just set up front and there's no attempt at calculating it dynamically.

That said, it's probably not a very costly operation either way.