Allow unique style for each instance

[billerickson]

Currently we have a site-wide style setting, used for every instance of the share buttons.

But in some cases you may want to use different styling based on the instance. For instance, above post, below post, and in sidebar.

We should add a $style parameter to the display() method here. If not provided, the site-wide setting will still be used.

We should also update the $locations array to include this setting. It will be blank by default (so site-wide is still used), but that will allow the style to be set using the ea_share_count_theme_locations filter.