Support for iterator-range parsing #290

Hi, it would be handy to add an overload to the parse method.

In my code, I deal a lot with vector<uint8_t> when communicating with my server.
However I sometimes have to send JSON, and I must convert the buffer to a std::string, which is unfortunate.

I know I could typedef basic_json and use std::basic_string<uint8_t> as StringType.
However, other parts of the code uses the std::string version, and I don't want to patch the whole code because of that.

We could add this overload to parse, which will behave like std::string iterator constructor:

template<typename InputIterator>
static basic_json parse(InputIterator begin, InputIterator end, parser_callback_t cb = nullptr);

// user code

auto buffer = receive_bytes();
auto json = json::parse(buffer.begin(), buffer.end());

What are your thoughts on that ?
If you agree with the idea, I can take care of the PR.