Automatic application name detection

[jeffkreeftmeijer]

In the Ruby integration, we automatically detect the application name from Rails applications. In Elixir projects, we should be able to default to Mix.Project.config[:app] as the project name.

WDYT @arjan, @tombruijn?

Related: #49.

[arjan]

Relates to #91 as well; Mix functions are not usable in runtime. However what you could do is update the instructions to let people put the following in the project's config.exs:

config :appsignal, :config,
  name: Mix.Project.config[:app],
  env: Mix.env

(untested, but should work)