GutterWidth and GutterHeight options

[MrGrabazu]

I add the support of gutterWidth and gutterHeight options.
If gutter option is provided so it will be dispatched to gutterWidth and gutterHeight.
If a class name is provided, I set the width css property value to gutterWidth option and height css property value to gutterHeight option.
Tell me what do you think about it.

[desandro]

Thanks so much for this contribution! This is tremendous work. But I'm a bit particular about the implementation. Having gutter, gutterWidth and gutterHeight may be confusing. I think being able to set gutterWidth and gutterHeight via an array would be more flexible

// single value for both gutterWidth and gutterHeight
gutter: 20
// use width of element for width and height (to maintain backwards compatibility)
gutter: '.gutter-sizer'
// separate values for width and height, set as array
gutter: [ 20, 10 ]
// element width for width, element height for height
gutter: [ '.gutter-sizer', '.gutter-sizer' ]

I appreciate all the word you did here. This issue has gained enough interest that this PR would make a great addition.

[MrGrabazu]

Yeah an array is a better idea :) and more flexible indeed !
I will use it in my fork, and add it to this pull request. Thank you for your support !

[mkhazov]

it's not available here, need to use this.options.gutter

[mkhazov]

@MrGrabazu thanks!
I made some changes in #396 so gutter option now can be an array.