add SVG sprite versions

[mojoaxel]

This adds support for SVG sprites so all flags can be loaded at once instead of one-by-one. This is useful if a lot of flags are used at the same time (e.g. in a country picker).

• Added example overview pages to show all 1x1 and 4x3 flags
• Added dependecy on svg-sprite to create a sprite of all flag images.
• Moved "common" SCSS styles from flagpack.scss to _common.scss, to also be able to use it in the sprites.
• Added npm scripts compile:sprite and compile to create the new files:
  • dist/flagpack_1x1_sprite.css
  • dist/flagpack_1x1_sprite.svg
  • dist/flagpack_4x3_sprite.css
  • dist/flagpack_4x3_sprite.svg

This is a draft on quite a big change, but it does/should not affect the existing functionality at all. Therefore I would consider this a minor change.

@jackiboy Please let me know what you think about this pull-request anf if you could image merging this. If not I'll simply create a new npm package flagpack-sprites or something similar.