Allow using several exception rule modifiers in a single rule

[ameshkov]

Currently, there's this limitation:

• urlblock, genericblock, generichide, elemhide, specifichide, and
  jsinject modifiers can be used only as a single modifier in a rule.

It's not clear why would we need it and it can be lifted.

For instance, in the case of mixing genericblock and generichide we can split it into two Safari rules.