Allow extracting fields based on attribute filters

[jakopako]

Right now, a field location has a selector among other config. This selector defines what node of the webpage should be extracted for the given field. Because of the nature of selectors you can define the node names (e.g. div, a, header ...) classes (with dot, e.g. .title) and id's (with #). You cannot specify values for other attributes, e.g. if you have the following node: <span style="color: #3366ff;">MATHIEU BOOGAERTS</span> you cannot filter on style="color: #3366ff;" just with the selector.

TODO: add a mechanism that allows such filtering.