Add initial_config option

If people only define an ENV var with an account-level push api key
appsignal is not aware of the app's name. With this initial_config
system the integrations can pass in a default app name that can be
overwritten in the config.
Bump to v0.8.3.beta.1 [ci skip]
Merge pull request #114 from appsignal/initial_config

Add initial_config option
Merge branch 'develop' of github.com:appsignal/appsignal-private into develop
Bump to v0.8.3 [ci skip]
Skip session sanitize if not a http request
Bump to v0.8.4.beta.0 [ci skip]