Add state_length option to specify the legth of state parameter

[tyamagu2]

I recently wrote a strategy for a service that only accepts state parameter shorter than 48 characters. The state length is currently hard-coded, so I had to overwrite options[:state] in setup_phase, but it would be nice if omniauth-oauth2 had an option to change state parameter length.

[coveralls]

Coverage decreased (-4.7%) to 80.247% when pulling 9520239 on tyamagu2:state_length into 464fcef on intridea:master.

[avokhmin]

maybe will be better to use

params = options.authorize_params.merge(options_for("authorize"))

and update

       def options_for(option)
         hash = {}
         options.send(:"#{option}_options").select { |key| options[key] }.each do |key|
           hash[key.to_sym] = options[key].respond_to?(:call) ? options[key].call(env) : options[key]
         end
         hash
       end

so, allow to use a Proc for generation of state

[avokhmin]

@tyamagu2 , see: #97

example with rails + devise:

  config.omniauth :instagram,
    'client_id',
    'client_secret',
    state:             Proc.new { |env| FooBar.state(env) },
    authorize_options: [:scope, :state]