support hierarchical indentation in select element.

Current codebase doesn't support indentation in the select dropdown for hierarchical taxonomies. For example, consider the following taxonomy:

Term 1
    Subterm 1.a
    Subterm 1.b
Term 2

Select dropdown will show up as:
Term 1
Subterm 1.a
Subterm 1.b
Term 2

This pull request mimics class Walker_CategoryDropdown from WordPress core to add indentation.