Added $optgroupsAttributes parameter to select method.

[djh101]

Addresses the concerns in #129. This PR does the following:

1. Adds a new $optgroupsAttributes parameter to the select method which is used to add attributes to optgroups. Previously attributes applied to a an array element that was an optgroup would apply the attributes to every option belonging to the optgroup (rather than the optgroup itself).
2. Increases the depth of $optionsAttributes by 1. To add attributes to options inside an optgroup, use the optgroup name as the key with an array of options and their attributes as the value. Previously there was no way to add attributes to options belonging to an optgroup individually.

See the formBuilderTest changes for an example implementation.