Allow for methods to *extend* components instead of replacing them

[sigmavirus24]

Right now the add_<component> methods will replace the existing value. We should probably allow for someone to extend these in an intelligent way.

For example: It might be nice to have an existing path, e.g., /python-hyper and allow them to add something "underneath" that, e.g., /rfc3986 such that the final path is /python-hyper/rfc3986.

Also it may be nice to allow someone to add to the query argument list instead of replacing it wholesale and interact with it as a list of tuples.