Support TaggedLogging, avoid use of initializer file

[PetrHeinz]

Wraps our logger in ActiveSupport::TaggedLogging by default.

Removes rake logtail:install.

Effectively reverts #11 as it broke installation process - after initialization, Rails.logger would contain instance of ActiveSupport::Logger. This PR recommends settings config.logger in config/application.rb to avoid inconsistencies with other logger gems.

Implemented Logtail::Logger.create_default_logger so that needed configuration is only a one-liner and we have some control over how Logger is instantiated by default (possibly to support other wrappers).

Tagged logging is supported in Rails 6.1 and higher

[PetrHeinz]

After merging, it needs:

• [x] gem release
• [x] docs update