Allow alterations to 'addthis_share' variable in 'addthis_configuration' alter hook

Problem/Motivation

There is an 'addthis_configuration' alter hook in the addConfigurationOptionsJs() method of the Addthis class. This allows alteration of all the 'addthis_configuration' settings. Currently, the only value you can change in the 'addthis_share' variable is that of 'templates'. There are other things that can be added to the 'addthis_share' variable - namely the url shortener options.

Proposed resolution

Merge alterations from the 'addthis_configuration' hook ($configuration['templates'] and $configuration['addthis_share']) into a variable that will be assigned to the JS variable 'addthis_share'.