Ability to specify some tiles

[TinyLittleEnormous]

Thanks for this great library ! I wanted to give a try to this method and your simple library / cli tool is just perfect for a newbie like me.
I have read more about WFC and found that is possible to fix some tiles in advance, and let randomness fill the rest.
This is an important feature for game maps (placing walls, chest, some authored content) , and will allow to generate images with better borders, by allowing only tiles that are on the sides of the input image on the sides of the ouput image.
I am refering to the part name 'fixed tiles' in this article :
https://www.boristhebrave.com/2020/02/08/wave-function-collapse-tips-and-tricks/

Thanks again for this amazing work !