Rebuild gem

[jgarber623]

This PR strips back most of the gem's functionality with the intention of relaying the foundation and reimplementing the CLI's functionality in a modern, streamlined way (toward a v5 release). A goal is to do this in a manner that's not terribly disruptive from the CLI angle, but that opens up more possibilities for using the gem directly via Ruby code.

This PR is the first step in that direction. The most relevant commit (c66f1e5) tells the biggest part of the story:

Changes include:

• completely refactored Ruby interface to the gem
• vendor-ing (thanks, MIT License) Active Support's very helpful "blank"
  extension

Note that many of the deletions to the CLI commands will return later.
This change lays the groundwork for rebuilding those aspects of the
project by first reimplementing the Ruby code in a modern, streamlined
manner.

require "svgeez"

Svgeez.build_sprite_sheet(
  "./app/assets/icons",
  "./app/assets/build/icons.svg",
  prefix: "custom-prefix"
)

The above example is a shortcut to an instance of
Svgeez::SpriteSheetBuilder (and its #build_sprite_sheet) method. If
you want or need a bit more control, you can also:

require "svgeez"

sprite_sheet_builder = Svgeez::SpriteSheetBuilder.new(
  "./app/assets/icons",
  "./app/assets/build/icons.svg",
  prefix: "custom-prefix"
)

sprite_sheet_builder.sprite_sheet
sprite_sheet_builder.sprites

sprite_sheet_builder.build_sprite_sheet

Important

This is a work-in-progress! I expect to add more functionality in the coming days and to update the project's documentation. What's here could change quite a bit before we get to v5.0.0.

Thanks for sticking around. 😄