Added the fromARGB and the fromRGB methods.

[MSeiler-png]

These methods may sound trivial, yet they can be useful for beginners which depend on a lsp for discovering the api. They typically search for creation methods and may not directly look at the fields.

[zenith391]

This is a good idea, I didn't consider discoverability with LSP

[Traxar]

The functions shadow variable names (->compile error) and there is a mistake with the position of the alpha parameter in the fromRGB-function. Please consider #55 to fix this.