Add a flag to tune the concurrency threshold

[junnlikestea]

vita/src/lib.rs

Line 109 in 2b237cf

const ACTIVE_REQUESTS: usize = 200;

Currently we use a hardcoded value to limit how many domains we can fetch data for at one time, but I feel like the user should have control of this value. For example, if I have a list of root domains which has over 10k entries, having a hardcoded value setting the limit to 200 might be capping me for no reason.

Instead we should add a flag with a default value set to something reasonable.

[junnlikestea]

Added in: bcd3c51