v0.5.4

Thanks to the following for their contributions:

• @acdenisSK

• @drklee3

• @Lakelezz

• @vityafx

• @zeyla

• [model] Add Message::member structfield (@zeyla) c:0e1e8fb

• [docs] Document example binding names for EventHandler method signatures
  (@acdenisSK) c:08a7110

• [model] Implement Mentionable for CurrentUser (@zeyla) c:4a24c90

• [model] Implement From<CurrentUser> for User and
  From<&CurrentUser> for User (@zeyla) c:af7f176

• [framework] Add option for bots to work only in certain channels (@vityafx)
  c:457a17e

• [framework] Differentiate in help whether a command is unavailable in DMs or
  guilds (@Lakelezz) c:89a18aa

• [framework] Improve Args docs (@acdenisSK) c:2603063

• [model] Add Message::mentions_user_id, Message::mentions_user
  (@Lakelezz) c:1162e68

• [docs] Update voice example 06 to make joining join the command invoker's
  voice channel (@drklee3) c:a80aab2

• [framework] Fix a framework example so it makes sense (@acdenisSK)
  c:63fe032

• [model] Remove deadlocking in Member::highest_role_info (@zeyla)
  c:c659bbd

• [framework] Dispatch to a threadpool only if required (@Lakelezz)
  c:23c5398

• [framework] Fix strikethrough behaviour (@Lakelezz) c:32c3bed

• [general] Fix links to the new repo location (@Lakelezz, @zeyla)
  c:152fe3d c:0324e01

• [framework] Switch to str::match_indices for some Args ops (@acdenisSK)
  c:cc6b567

• [framework] Remove if length == 1 branch in Args functions (@acdenisSK)
  c:6346975

• [framework] Optimize Args::find, Args::find_n (@acdenisSK) c:5ba521b

• [framework] Revamp Args from the ground up (@acdenisSK) c:ff9edc0