New time parameter

[rschirin]

Hey there,
I have modified the official master branch with a custom release. Basically, I modified the section related to span documents over a window time. Since I had the need to distribute documents in a smaller window time (i.e. only 10 minutes) I implemented this feature. Maybe it could be helpful also to someone else.
The command line follow the example below:

docker run --rm makelogs --url "https://elastic:xxxx@0.0.0.0:9200/" --indexPrefix "test-" -c 300 --time "30m/2h" --no-reset

I haven't any particular JS experience, so I modified the code just to get my purpose avoiding any noticeable error.
Just a fast recap:
-d classic behavior with default value if no value has been specified (1 day before and 1 day forward)
-t new behavior with default value if no value has been specified (1 day before and 1 day forward)

In a specific way:

-t 2 (2 days before till today)
-t 5h (5 hours before till today)
-t 1/2 (1 day before and 2 days forward)
-t 3h/50m (3 hours before and 5 minutes forward)