sprite.yml

[naogify]

Related to #36 (comment)

Regarding sprites, are we using file names of the .svg files as the name of the sprite?

We might want to think about sprite.yml file which assign sprite name to each sprite graphics. sprite.yml might look like:

airport: https://example.com/somewhere/airport.svg
branch: ./icons/gsi-icon.svg

I think sprite.yml will be helpful because it would always be good to optimize the size of sprite file. By introducing sprite.yml we can promote nicely customized sprites.