--find-links option should have multiple=True

When doing `pip-compile -f foo`, the find_links argument to compile (https://github.com/nvie/pip-tools/blob/future/piptools/scripts/compile.py#L158) is a string, not a list.  When you do `extra_find_links.extend("foo")`, extra_find_links ends up being ['f', 'o', 'o'] instead of ['foo']. The `multiple=True` argument to click.option tells it to pass a list instead of a single string.