Simplify JSON/HTML report generation

[fabricereix]

Currently, an html report can be generated easily with --html DIR.

But in order to accumulate results for different hurl executions, a json file needs to be used with the option --json.
This makes the options --json and --html not symmetrical, and thus, this is not really intuitive.

In order to accumulate results into an html report, the html report itself should be appended directly, without the need to use the option --json.

The option --append can also be removed. Results can be naturally appended to the JSON/HTML report file if this one already exists.