Add configuration for location of strings in Scanner

[Arzaroth]

This addresses both #208 and #143

I didn't provide tests for this PR since there is no concrete implementation of CodeScanner in this repository

[oscarotero]

Hi, thanks for your work with these issues!

I'd rather have these two issues in two different pull requests, because I have some doubts:

• I'm agree with adding the option to enable or disable the translations locations, but I don't like the option name because it's confusing. I'd rename to addReferences(), to keep consistency with the $translations->getReferences() function.
• I'm not sure about the relative path, and this is why I'd like to have a different pull request. The reason is that perhaps it add too much responsabilities to a class that is created to scan an unique file. If anyone want to use a different file path, the solution is simple: passing directly the content and filename. For example:

$contents = file_get_contents($file);
$relativePath = get_the_relative_path($file);

$translations = $scanner->scanString($contents, $relativePath);

This function is already present in the interface (https://github.com/php-gettext/Gettext/blob/master/src/Scanner/ScannerInterface.php#L21) and this way is much more flexible, so I'd wouldn't add this feature (at least, for now).

[Arzaroth]

Points taken.

I'll close this PR and submit two new ones.

Regarding relative paths, I didn't realize what you suggested was possible. It is indeed a nice workaround and I understand your argument on this matter.