support combining multiple result files into a single one a'la coverage

[zsoldosp]

it was out of scope for #21 which already turned out to be quite big

the suggestion is to have a djpt_combine_result_files command that takes filepaths as *args (LabelCommand) and reads all of them to memory, then write it out into a single file named django_performance_testing.serializer.get_datafile_path

The idea behind using labels instead of accepting file patterns and then converting them to filepaths based on glob or re or similar is to keep things simple here.

people running things in parallel would be expected to have different file names for each run in their settings (e.g.: djpt.results_collected.$$ to get the running process's id), and then do the bash magic themselves (e.g.: manage.py $(ls djpt.results_collected*)). Probably this should be documented :)