support for boolean values in `:ignore` config key

[ktravers]

currently, to ignore all errors in a specific environment (such as :test or :dev), you set :ignore config to a simple function that returns true:

config :airbrakex,
  ignore: fn(_error) ->
    true
  end

it'd be more concise / convenient to allow users to set a boolean value here instead:

config :airbrakex, ignore: true

i have a branch open with the change. let me know if this is something you want to support, and i'll be happy to open a PR.