Format option and Logfmt support

[Unactived]

This PR adds an optional --format option that defaults to the current console format, and adds a logfmt logger with it.

Partially addresses #3

[Frky]

Thank you for the PR.
To reply to the question in #73, it is of course very fine by us if you want to PR something into masscanned, and even more if it addresses an issue (you or someone else has), so feel free.

Just two remarks before it get merged:

• could you run cargo fmt on your code?
• not to be picky, but is there a reason for et instead of eth for ethernet in the key you chose (for instance, https://github.com/ivre/masscanned/pull/74/files#diff-cda9af6f45f28c2fc5065bbcaa066c8273112acd55be28853780b9968f9b450aR167 but not only)? Is it to be compliant with a tool that ingest logfmt files? Or is it a personal choice?

Other than that, I'd be happy to merge your PR.

[Frky]

Could you run cargo fmt on your code?

[Unactived]

* not to be picky, but is there a reason for `et` instead of `eth` for `ethernet` in the key you chose (for instance, but not only)? Is it to be compliant with a tool that ingest logfmt files? Or is it a personal choice?

I personally have no problem with it being eth or whatever else. (I guess some might appreciate having only one instance of lowercase eth though).
I actually meant it as short for EtherType (acronym ET, but staying lowercase), which is the name of the field.

[Unactived]

I think I'll change et to eth_type which is indeed clearer without looking at the code or thinking about the protocol's details.

[Frky]

Thank you for the changes (and for the PR once again).