Customise the timeout

How would I go about customising the timeout? I'm not sure if I should alter https://github.com/mafintosh/block-trace/blob/master/bin.js#L33 or https://github.com/mafintosh/block-trace/blob/master/bin.js#L53 or both?

It would be great to be able to set BLOCK_TRACE_TIMEOUT as an environment variable and have it read from defaulting to 1000 if it wasn't set?

```
var timeoutEnv = Number(process.env.BLOCK_TRACE_TIMEOUT),
timeout = Number.isNaN(timeoutEnv) ? 1000 : timeoutEnv;
```

The timeout handling is in bin.js at lines 33 and 53; start by comparing those two uses and how the command currently obtains its timeout. Done means BLOCK_TRACE_TIMEOUT overrides the timeout, while an unset or non-numeric value retains the default of 1000.

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