Fixed data was truncated on a property value longer than 255 chars

[andy1xx8]

Hi. If the data is longer than 255 characters (hardcoded in the source code), it will be truncated. This is a really serious problem because the URL from Google Ads is longer than 255 and was mentioned here #405

Therefore, this pull request will allow you to specify this limitation via config while constructing the mix panel tracking lib.