* Make exception context optional

Change exception context parameter to pointer and replace context with
nullptr where appropriate.

* Support escaping other string types

* Add string concatenation function

Add variadic concat() function for concatenating char *, char, and
string types.

* Replace string concatenations using + with concat()

* Template json_pointer on string type

Change json_pointer from being templated on basic_json to being
templated on string type.

* Add unit test for #3388

Closes #3388.

* Fix regression test for #2958

* Add backwards compatibility with json_pointer<basic_json>

* Update json_pointer docs

* Allow comparing different json_pointers

* Update version numbers