Support custom patching logic

[krzemin]

Sometimes, when patching a case class, new value must be combined from existing value and provided one from patch. Thus, we need some way of expressing the custom logic in patchers DSL, as in the example from this comment: #98 (comment)

[MateuszKubuszok]

Depends on #538 which in turns depends on #115.

[MateuszKubuszok]

The functionality is available on master, see testing-snapshots if you want to help us testing it (and checking if the documentation makes sense) before we'll release it (I want to do a few much smaller things before the release).