Implement speed reporting in the progress bar and the --report-speed switch

Original discussion at !328 (closed)

Fix #317 (closed)

• docs/wget2_manual.md: Add documentation for the --report-speed switch
• include/wget/wget.h: Define the report speed types, add reference to wget_bar_set_speed_type()
• libwget/bar.c: Modify the wget2 progress bar to print speed information
• src/bar.c: Add code to set the type of speed reporting
• src/options.c: Add the --report-speed switch and a function to parse the legal types
• src/wget_options.h: Add 'report_speed' in the config structure
• src/wget.c: Print speed reports at the end of a download thread