In contrast to using http_responsecode and http_loadtime with the same
configuration, this plugin performs only one request per site and munin
run to gather its statistics.

- ignore include during shellcheck
- check if needed programs are installed
- use long argument names for graph_args
- remove un-needed empty definitions
- improve variable names
- validate parameter max_time
- update copyright statement
- write all configurable arguments into one array
- use same check for short_label and follow_redirect
- reformat documentation