Adding language to the context when using Airbrakex.Plug

[coryc5]

Hello,

We use Airbrakex.Plug for tracking our Elixir errors and we would like to include in the context of the notice that this is an Elixir error. When the Plug calls Notify.notify/2, it does not pass a context, so it seems that this is the function head that gets hit:

https://github.com/fazibear/airbrakex/blob/master/lib/airbrakex/notifier.ex#L38-L40

We would like in those cases for the context language to be set to "Elixir". Would you be open to a PR that makes that change? Or would you prefer that be configurable in some way?