Added Blocks support, as well as adjusted for new SearchSort enums returned by the Slack LoginResponse model

[PeterJFerrarotto]

Added support for Blocks to all API methods, and also accounted for two new enum values (not_set, relevant) for SearchSort from the Slack LoginResponse -> Self -> Preferences model. (Relevant seems to be solely used for search_channel_sort and search_people_sort [two new SearchSort values in the response that I also accounted for], but it's possible it could be set to that for the overall search_sort as well.)

[Inumedia]

There's already support about to be added for Blocks by @clockworkcoding @ #172