Add support for custom builds

[LitoMore]

Resolves #163

Changes

• Add support for custom builds
• Use getIconSlug() from simple-icons/sdk

Example

SI_FONT_SLUGS_FILTER=github,simpleicons npm run build
#=> github      \u{EE3B}
#=> simpleicons \u{F3FE}

The SI_FONT_PRESERVE_UNICODES is useful when users want to use only a few icons, and the Unicode doesn't change between version updates.

SI_FONT_SLUGS_FILTER=github,simpleicons SI_FONT_PRESERVE_UNICODES=false npm run build
#=> github      \u{EA01}
#=> simpleicons \u{EA02}