Missing document about the parameter options.dirty

[Frimaire]

There is an interesting option dirty in the signal() function. option.dirty is a callback, which determines if the new value should be treated as a value different from the previous. The default one is !==.

It seems that this option has not been documented in README. A brief description would be helpful.

Actually, I found this option when I was trying to patch the library in other to give a option, which makes the signal to trigger the effect whether the value actually changes or not. Overriding the _change with returnTrue is a good way to do that.