Option to configure flamegraph sampling rate

Sometimes it's useful to be able to control how precise the flamegraph generate by rack-mini-profiler is.
This pull-request provides two ways to do that:
* Permanently, using the `flamegraph_sample_rate` configuration option
* For a single request, using the `flamegraph_sample_rate` param in the query string

I kept the same value as before by default, and updated the docs accordingly.