Add optional parameter parsing for HTML file name

[ktchani]

Added a feature for adding optional parameters to the HTML report file name.

Here is an example:
locust --headless -u 12 -r 10 -t 10s --host http://127.0.0.1:8000 --html "{u}_{r}_{t}_locust.html"

This would generate a filename:
12_10_10_locust.html

I use locust a lot at work, and in order to keep track of various test conditions among all the report files, I would manually update the filename. With this change, the file names can be more indicative of what the test was for. I would also consider adding P50, avg, etc...