refactor: pass srcset modifiers through the options parameter

[sherwinski]

This PR refactors how the new srcset modifier options (see #54, #55, and #56) are required to be passed to Imgix::Path#to_srcset. Instead of passing values directly through widths, width_tolerance, min_srcset, and/or max_srcset keyword parameters, users should now store any combination of these arguments in the new options parameter as a hash.
This change is being made to avoid confusion in naming between imgix URL parameters and parameters that modify object behavior. This should hopefully clarify through design of the interface which options modify how srcsets are generated versus how the image itself is modified through imgix's service.

Example:

client = Imgix::Client.new(host: 'testing.imgix.net', include_library_param: false).path('image.jpg')
client.to_srcset(options: {min_width: 500, max_width: 2000, width_tolerance: 0.20})

Will output the following:

https://testing.imgix.net/image.jpg?w=500&s=e39b3eddc94ae9f21c2d5ed5772c788e 500w,
https://testing.imgix.net/image.jpg?w=700&s=9c76f8c8fde1de38ddf85c2472ca6289 700w,
https://testing.imgix.net/image.jpg?w=980&s=2416cecd450f40f4a31b6af233d47d91 980w,
https://testing.imgix.net/image.jpg?w=1372&s=73ddbeace48bcb265ea1f24d2111e7e8 1372w,
https://testing.imgix.net/image.jpg?w=1920&s=0a3a9652aeae2262950c22cef2d97adf 1920w,
https://testing.imgix.net/image.jpg?w=2000&s=e51013d2c46f98c0f1b845a042d4caf2 2000w