Add an override for the UN flag 1f1fa-1f1f3.

[KwanEsq]

Fixes #28.

Step-by-step creation instructions:

• Copy twemoji/2/svg/1f1fa-1f1f3.svg to twemoji-colr/overrides/1f1fa-1f1f3.svg
• Open twemoji-colr/overrides/1f1fa-1f1f3.svg in Inkscape
• Select the path with the landmasses in (7th of the 9 paths in the group)
• In the "Path" menu select "Break Apart"
• Several of the ocean sections are now solid white
• Select one of the new paths making up a white ocean section
• Shift-click every white ocean section, so they are all selected at once
• Open twemoji/2/svg/1f1fa-1f1f3.svg in Inkscape
• Flick between the modified and second copy to compare and not miss any small bits
• In the "Fill and Stroke" > "Fill" panel click on "Flat colour"
• Use the colour picker to set the background blue as the fill colour
• Double check you didn't miss any small bits
• In the "Object" menu select "Group"
• "File" > "Save As..."
• Choose "Optimised SVG" as the output format and "Save"
• Tick the following options:
  • Options
    • Shorten colour values
    • Convert CSS attributes to XML attributes
    • Collapse groups
    • Create groups for similar attributes
    • Work around renderer bugs
  • SVG Output
    • Remove the XML declaration
    • Remove metadata
    • Remove comments
  • IDs
    • Remove unused IDs
• "OK"
• Optional: open the override SVG and remove the version attribute from the root svg element

[KwanEsq]

@jfkthame I'm happy to actually submit this now I've documented how to do it.

[jfkthame]

Thanks, especially for including such thorough notes!