Support multiple transformations per selector

[danielytics]

Currently each selector can only contain one transformation. Since selectors are map keys, you cannot have multiple identical selectors access the same node, so the only way to apply multiple transformations (eg setting content and classes) would be to use different selectors that happen to access the same node (see #8). It would be useful if an ordered list of transformations can be (optionally) supplied to a single selector, which are then applied to the addressed node(s) in order.

This feature is needed by Erinite/template-stylesheets.

[danielytics]

Will implement as soon as #10 is resolved.