Detect and map to existing product attributes on import

Currently when setting a field to use the "Custom Attributes and Post Meta > Custom Field / Product Attribute" option, the imported product attributes are always added as new attributes, instead of mapping to existing attributes.

This should fix that (#50).  It uses existing product attribute if the "Custom Field Settings" name matches the attributes slug (case insensitive).

So, for anyone else using this fix, you should always use the attribute slug when specifying the "Custom Field Settings" Name for the column so that'll match to the correct product attribute.  The attribute terms in your imported data can be either a existing term name or a term slug.