AppendQueryParam(s) methods

[tmenier]

Add methods similar to SetQueryParam(s), except that never overwrite.

Example using SetQueryParam:

"http://url.com"
  .SetQueryParam("x", "1")
  .SetQueryParam("x", "2")
  .SetQueryParam("x", "3");

// result: http://url.com?x=3

Same using AppendQueryParam:

"http://url.com"
  .AppendQueryParam("x", "1")
  .AppendQueryParam("x", "2")
  .AppendQueryParam("x", "3");

// result: http://url.com?x=1&x=2&x=3

This will also form a basis for fixing #370, which on its own is a bit involved, and would be better bang for the buck to expose this functionality more generally.

[tmenier]

Now available in prerelease 4 https://www.nuget.org/packages/Flurl/4.0.0-pre4