Add go.mod

Updated entities and json structs for API v2.6.0

Added missing fields to various structs and sorted them in the same order as
the original mastodon API documentation does. This should make it easier to
compare go-mastodon's structs with the original documentation.

also convert to string in attachments, pagenation

Merge pull request mattn#56 from 178inaba/stream_error_handling

Fix not to return error event of context in streaming