Added a walk option

[romaricpascal]

I needed a way to customize which nodes were walked through so I added a walk option allowing users to customise the walking (eg. use walkDecls instead of just walk).

[andyjansson]

I really don't get this change.

[romaricpascal]

I'm making a plugin that processes url() declarations in the CSS through a url function. However, I want it to apply only to images urls, so for example ignore the ones in @font-face declarations.

This is why I added the walk option to allow me to override the default css.walk() call going through everything. This way, I can use walkDecls (and a RegExp) to only target the properties accepting an image URL according to the CSS specs.