Allow rewrite endpoints to specify a different query variable name

[johnbillion]

Using add_rewrite_endpoint() we can add an endpoint such as /foo/bar/ which maps to foo=bar. I'd like to be able to map an endpoint to a query variable of a different name. Currently the query variable name always matches the endpoint name.

Example: I'd like to use /p/{number} as a nice short endpoint for some custom functionality, but the 'p' query variable is used by core. I'd like to be able to map /p/3/ to my_query_var=3.

Patch which adds an optional third parameter to add_rewrite_endpoint() for specifying the name of the corresponding query variable. Defaults to the endpoint name.