update create_dxfs_style

[JanMarvin]

This PR changes the default behavior for create_dxfs_style(). We only create a dxfs style with style information the user requested. Previously we would ship a default font type, font size, font color and background color.

Now this

create_dxfs_style(
  font_color = wb_colour(hex = "FF9C0006"),
  bgFill = wb_colour(hex = "FFFFC7CE")
)

creates this

<dxf>
  <font>
    <color rgb=\"FF9C0006\"/>
  </font>
  <fill>
    <patternFill patternType=\"solid\">
      <bgColor rgb=\"FFFFC7CE\"/>
    </patternFill>
  </fill>
</dxf>