Show results

[alexfernandez]

Add function to show result. Use with result.show().

Also in this PR:

• Improve readme: show version with await loadTest().
• Do not mutate options parameter to the API, instead create a new options object.
• Fix API option requestsPerSecond, which was completely broken.
• • Fix API option keepalive which was also broken.
• Convert some error traces to proper exceptions when parsing options.
• Properly shut down when requestsPerSecond is used and takes less than a second.
• Separate latency results into its own file.
• Fix callback error for loadTest(), was being passed as second parameter instead of first.

For immediate merge, to be released as v6.2.0.