add get_customer_count method accept filters on get_customers method
add get_customer_count method
accept filters on get_customers method

garrinmf/bigcommerce-api-ruby,sgerrand/bigcommerce-api-ruby,hasclass/bigcommerce-api-ruby,YotpoLtd/bigcommerce-api-ruby,mancrates/bigcommerce-api-ruby,mattolson/bigcommerce-api-ruby,emeraldMaster/bigcommerce-api-ruby,gregory/bigcommerce-api-ruby,bigcommerce/bigcommerce-api-ruby,YotpoLtd/bigcommerce-api-ruby,RyanSacha/bigcommerce-api-ruby,pedelman/bigcommerce-api-ruby