Allow for optionally specifying the speedtest server ID

[lucymhdavies]

For whatever reason, I've found that the nearest speedtest servers cap out at about 100Mbps down, but some others I'm able to get upwards of 550Mbps down (which therefore feels more accurate).

As such, being able to specify which specific server ID I want to use would be useful.

Doesn't look particularly complex to implement:

• A new flag here - https://github.com/caarlos0/speedtest-exporter/blob/main/main.go#L18-L24
• Optionally specifying the -s ID parameter to the exec.Command

So happy to provide a PR for this :)

[lucymhdavies]

First pass of this in https://github.com/caarlos0/speedtest-exporter/pull/11