add term-multiselect arg filter, and placeholder support

Currently term-multiselect only shows placeholder as "Choose a category" and does not support setting placeholder, even if using term-mutliselect for something other than job category.

The reason I added the code the way I did was to still support the default placeholder "Choose a category", if you pass placeholder as empty, select2 will automatically set placeholder to "Choose an Option".

With the code structured this way the user can just use a space for the placeholder if they want it blank, leave the field blank for using default, or set it to whatever they would like.

Also added a filter for the arguments for any possible future changes