support for http headers

[martinduris]

After this pull request we can use:

TranslateHttpBasicAuthLoader(http, new HttpHeaders().set('Authorization', 'Basic ' + btoa(conf.username + ':' + conf.password)), environment.base_path + '/i18n/', '.json');

[ocombe]

Hello, thanks for the PR but I prefer to keep this loader minimal. If anyone wants this functionality, it's super simple to copy your code and use this instead, that's why I made the library modular, there will always be someone who needs a new thing on their loader.

[martinduris]

@ocombe hi, i understood - thanks for reply !

[woteska]

I also understood, but I don't understand, where don't you have to set headers for a http request. :) So I think it should be in basic implements, but of course, I can make a little workaround... just it is not logical.

Thank you for all of that!