Support Reading/Writing PNG tEXt, zTXt, and iTXt Chunks

[bobbah]

@JimBobSquarePants requested I open this feature request.

As discussed in Glitter, the library currently cannot write (and does not expose reading) PNG tEXt, zTXt, or iTXt chunks that are used for storing data in the PNG format.

It would be a helpful feature as in some implementations of using the PNG format these data chunks are used to store critical information for consuming the file in various applications.

As posted in Glitter by JimBobSquarePants, the specification of these chunks can be found here:
https://www.w3.org/TR/PNG/#11zTXt

[JimBobSquarePants]

Note to self: We'll need to put some thought as how to store the metadata. I don't think the ImageProperty property collection will suffice. (I'm not even sure if we should keep that property.)

[brianpopow]

I think i can do that. @JimBobSquarePants why do you think the ImageProperty would not be sufficient?

[JimBobSquarePants]

Fixed with #951