Container-policy parameter for parse functions

[psiha]

Consider adding the option to provide a policy type parameter that specifies which containers the library should use by default (to minimize conversions and/or minimize verbosity - need for the user to specify his own preferences) like

struct containers
{
  template <typename T> using vector = boost::container::small_vector<T,...>;
  template <typename Key, typename Value> using ordered_map = boost::container::flat_map<....>;
};