Add 'cropSymmetric' to AutoCrop

[f-barth]

Why?

• I have a use-case that needs autocropping, but in a symmetric way. That means if I crop 2 pixels from the left I also want to crop 2 pixels from the right to keep the image centred.

What?

• Add configuration of autocrop via options object: The parameters to autocrop were parsed "by-type" which did not allow adding a second boolean flag. I added the option to configure autocrop via an options object.
• Add symmetric autocrop: if enabled "horizontal crop = Math.min(left crop, right crop)", same for vertical
• Add tests for the two above.

Notes:

• Community Maintainer Guidelines #223 mentioned the idea of using option objects instead of maaany parameters. I took up the idea. autocrop still is does handle the "old" invocation via parameters. All "old" parameters and symmetric autocropping can also be configured via options object.

[hipstersmoothie]

@edi9999 This one is good to go now too.