[WIP] feat: add --iterations flags to specify how many times to run target binary

[bartlomieju]

Hi there! Thanks for the great project, we use it extensively at Deno.

While we are working on improving our startup time, we found out that running the target binary just once is not enough to collect reliable data to see where we're spending time. So I added --iterations flag that allows to run target binary multiple times and create a single flamegraph out of all the runs.

This is a quick a dirty implementation, but it allows us to collect more detailed data. If there's an interest in adding this flag and upstreaming the patch I'll be happy to clean it up.