Cannot specify compresslevel on save

[bcipolli]

Gzip allows a compression level. ImageOpener can handle a compresslevel parameter. However, save, to_filename, and to_file_map do not expose this to the user. Seems the only way to do this is to change the default_compresslevel

load does allow the user to specify optional arguments; seems it would be useful to do for save...

This is preventing me from speeding up creation of large images for testing, but I could imagine someone wanting to vary the compression level (higher or lower; default is minimal, after all).

Feel free to close as wontfix or let me know if I'm missing the way to do this.

[matthew-brett]

Seems reasonable to me, if you have a use for it...

[effigies]

Relevant comment: #566 (comment)