Ability to ignore image (e.g. SVG)

[ldanielswakman]

@jancbeck Would be useful if the plugin has the ability to pass an ignore parameter to not apply the srcset/responsive images behaviour. This would be especially useful if you have SVG images (where srcset doesn't make much sense), or image instances that are always meant to be high/uncompressed quality.

This could be done in multiple ways:

• [x] Adding an responsive-ignore: true parameter to the kirbytag: (image: company-logo.svg srcset-ignore: true)
• [x] Adding a config rule to ignore certain filetypes: c::set('responsiveimages.ignore-ext', array('svg', 'gif'))

Suggested fix: https://github.com/jancbeck/kirby-responsive-images/pull/14