Explain options in usage

See merge request bortzmeyer/homer!17

Options' output refactored

See merge request bortzmeyer/homer!16

When using monitoring, the exit call is part of the print_result
function, making these last tests before exiting unnecessary

HTTP/2 streams

Closes #26 and #8

See merge request bortzmeyer/homer!11

Remove short option for debug

See merge request bortzmeyer/homer!15

Short option -d is already in use as the short option
for --delay

Option debug

Closes #25

See merge request bortzmeyer/homer!14

Calculate time for each requests.
Closes #26

See merge request bortzmeyer/homer!13

These tests use curl output to assess if the behavior is
right or not.
However, the curl output can not be reliable in the current
situation where we look for a multiplexed connection, but
curl might have decided to reuse the connection if it is
available.