Customizable Letter

As requested by [hetile](https://www.thingiverse.com/hetile/about), a customizable "letter paper". You can use it for invitations to birthdays and more. The letter paper width and height will automatically adjust depending on the text and font size chosen. In case the paper width is too small or big, adjust "adjust_width_factor".

By default, the letters on top are .4 mm high for contrast. The paper itself is just .2mm, so that it is one layer if you print at .2mm layer height.
Make sure to