Add support for writing ASCII flavours of PNM formats

[mloskot]

Currently, only binary PNM files can be written.
We need support for ASCII flavours too.

This is necessary to enable hosting of test images in human-friendly textual format that allows to easily craft any test data and verify actual vs expected outputs.

References

• Clarify what flavours of PNM formats can be read and written #358
• Discussion on [RFC] Managing input images for tests
• Discussion on [IO] Hosting test images

[psbro]

i want to contribute to this issue.would you exlain it to me?

[mloskot]

@psbro I think the task in this issue is self-explanatory (with help from reading the linked references, GIL I/O docs and source code and other online matrials about the format in question, of course) but if you have any specific questions feel free to ask.