Add blessing priorities.

[JerryLingjieMei]

#347 Automatically select the blessings with the highest rarity within each priority group.

[LmeSzinc]

Not accepted. Filters parsing must follow the given filters, no rarity priority should be added if filter don't have a rarity priority.

[JerryLingjieMei]

Not accepted. Filters parsing must follow the given filters, no rarity priority should be added if filter don't have a rarity priority.

Under most circumstances, higher rarity blessings are usually better than lower rarity blessings when under the same filter group. For example, the user may not want to specify explicitly that nihility-3>nihility-2, hunt-3>hunt-2, and random-3>random-2, etc.; they only need to state nihility>hunt>(champion dinner)> reset>random.

[LmeSzinc]

This is a code quality thing, business code should be separated from non-business code. Filter class is a general class for filter string parsing, there shouldn't be a new argument added just for one business scenario.