Implement set/get methods for api keys instead constants

[shakaran]

As per doc and exception messages show, it recommends to configure the ApiKeys.php files for example for use Mozscape service with the ACCESS_ID and secret code.

The problem with this if you install through composer it will generate in vendor/seostats/seostats/SEOstats/Config/ApiKeys.php a file, if I edit the file, it could be overwritten in future with new SEOstats versions and I cannot set dinamically in code for different mozscape keys since it is treated as a constant.

So, it would be nice if we have a get/set method in api keys, so it could be configured in execution time.

[eyecatchup]

You can now use DotEnv for ApiKeys. See #131

[shakaran]

@eyecatchup this is a better solution, but it doesn't satisfy really my issue. When I want use several API keys in execution time it always will load the same .env file or I am forced to rewrite the .env file in execution time, so for me this change is not really solving my issue

[eyecatchup]

OK, get it. I see what I can do.