feat: add ability to pass in custom widths

[sherwinski]

This PR adds logic that allows users to pass in a custom widths list when generating srcset pairs.
In certain cases when users know exactly what widths they want to have their images rendered at, they will be able to pass in an array of integers into the widths: keyword parameter.
See the following example:

@client ||= Imgix::Client.new(host: 'testing.imgix.net', include_library_param: false)
.path('image.jpg')
.to_srcset(widths: [100, 500, 1000, 1800])

Will generate the following srcset of width-pairs:

https://testing.imgix.net/image.jpg?w=100 100w,
https://testing.imgix.net/image.jpg?w=500 500w,
https://testing.imgix.net/image.jpg?w=1000 1000w,
https://testing.imgix.net/image.jpg?w=1800 1800w

Please note that in situations where a srcset is being rendered as a fixed image, any custom widths passed in will be ignored. Additionally, if both widths and a width_tolerance are passed in to the to_srcset method, the custom widths list will take precedence.