add join function

[WimLeflere]

Prototype join function (#199), what do you think?

Test and documentation not yet added.

This code

#include <iostream>
#include <vector>

#include <inja/inja.hpp>

int main()
{
   auto data = nlohmann::json();
   data["guests"] = std::vector{ "Jeff","Patrick","Tom" };
   data["numbers"] = std::vector{ 1,2,3 };

   std::cout << inja::render("{{ join(guests, \" - \") }}", data) << '\n';
   std::cout << inja::render("{{ join(numbers, \" | \") }}", data) << '\n';
}

Generates this output

Jeff - Patrick - Tom
1 | 2 | 3

[pantor]

Looks good to me! Do you want to add a few tests to test/test-function.cpp?

[pantor]

Cool, thanks so much!