Make main file accept comma-separated values and turn them into arrays.

The purpose of this commit is to make the plugin accept CSV entries like this:

"user_login","user_email","user_pass","first_name","last_name","display_name","favorite-things","user-groups"
"testuser","testuser@localhost.local","pwd","Test","User","Test User","83,84,85","fanatics,dudes"

resulting in the creation of an array for 'favorite-things' and another for 'user-groups', which will be correctly inserted in the database as serialized data. Taking the "favorite-things" field, for example:

[83,84,85] => Inserted into wp_usermeta with meta_key = "favorite-things" and meta_value = "a:3:{i:0;i:83;i:1;i:84;i:2;i:85;}"