Inserts a tEXt chunk into a given png image.
The following args should be passed to the executable:
- The input directory of the png image.
- The output directory of the png image.
- The keyword for the tEXt chunk. (See 4.2.3.1. tEXt Textual data here)
- The text to include, relative to the keyword.
- Assumes that the tEXTt chunk should be added at n-12 bytes of the png file.