GetLanguageByContent does nothing

[erizocosmico]

In GetLanguageByContent (https://github.com/src-d/enry/blob/master/common.go#L88) we can see the passed filename is "", but in GetLanguagesByContent (https://github.com/src-d/enry/blob/master/common.go#L382), which is called by the aforementioned function, returns always nil if the extension is not matched, which always happens because GetLanguagesByContent is explicitly passing an empty string.

We should either fix this or remove this exported function, because it does nothing.

[abeaumont]

Thanks for reporting @erizocosmico.

[abeaumont]

It doesn't make sense for this function to not have a filename parameter (even the README.md has an example where the parameter is there). I'll add that parameter.