CLI error message is unclear when `--` is in wrong place

[vi]

Describe the bug

Error message about wrong CLI usage is unclear

To Reproduce

$ texture-synthesis --out-size=171x248 --out=w.png -- generate q1.png 
error: The subcommand 'generate' wasn't recognized
    Did you mean 'generate'?

If you believe you received this message in error, try re-running with 'texture-synthesis -- generate'

USAGE:
    texture-synthesis --out-size <out-size> --out <output-path>

For more information try --help

Expected behavior

Error message helps me to fix the command line, not to substitude generate for generate.

The command line works if -- is removed. As -- is often present in README examples, probability that users would step on this is higher.

[Jake-Shadle]

Yah, that's kind of an unfortunate suggestion, but it appears like is no simple way to fix this particular suggestion without turning suggestions off altogether.

However, the use of -- in the README is actually unnecessary with the current arguments, so I will just remove the unneeded usages of that, which should avoid (most) confusion.

[vi]

Another CLI UI strangeness:

        --out-fmt <out-fmt>                       
            The format to save the generated image as.

            NOTE: this will only apply when stdout is specified via `-o -`, otherwise the image format is determined by
            the file extension of the path provided to `-o` [default: png]

Explicit command-line option should override any sort of auto-detection.