Moves ct_extra_params to the end of the generated ct_run command

This patch places `ct_extra_params` at the end of the generated `ct_run` command. This allows users to use `-erl_args` at the end and pass options directly to the emulator.

The integration test includes an example of how to pass a `ct_run` option followed by an `-erl_args` option:

`{ct_extra_params, "-repeat 2 -erl_args -config app"}.`

This should fix https://github.com/rebar/rebar/issues/271