Add 'boost::intersperse' string algorithm

[denzor200]

Analague of ranges::views::intersperse from range-v3 library.

const std::string input{"foobarbaxbat"}; std::cout << boost::intersperse_copy(input, ',') << '\n'; // Outputs: f,o,o,b,a,r,b,a,x,b,a,t 

[jgopel]

👍 LGTM - @mclow will have to approve.

[denzor200]

@mclow hello, would you like to review this PR?