Add support for optional LANG param

[timurgarif]

#16

[alexisvisco]

Hey thanks for your pull request, can you provide a go enum of the language supported by sonic please ?
It will be nicer for the developers :)

[timurgarif]

Oh, good point! I didn't think of it. The only drawback of this approach would be need to maintain the enum every time Sonic adds/removes support for new languages.
I'll do that a little bit later.

Also the PR introduces the public API change. The package has no releases yet (no stable sem versions). So it should be OK?

[alexisvisco]

It's not a big deal, developers need to be agile, I am not active on the project so there are some consequences haha :)

[alexisvisco]

For maintaining the enum not really, you can override or add an enum since it's a string.

[alexisvisco]

If you really want to not introduce a breaking change you can add a Query() without the lang and a QueryWithLang()
Not optimal but it's a solution.

[timurgarif]

Nice. Then we'd better to stick to the current API to keep it closer to the original Sonic protocol.

BTW, I amened the PR as you suggested.