Fix #55

Only drop the last empty component. The new behavior has the following effects:

1. The first empty component is preserved and can be treated specifically as
   requested in #55. The assumption is that a separator at the beginning
   is intentional.

   (cdr (orderless-compile " ")) => ("")
   (cdr (orderless-compile " foo")) => ("" "foo")

2. Completely blank input will compile to an empty regexp list. This is
   important for completion command startup performance:

   (cdr (orderless-compile "")) => nil

3. When separating components, the last separator will not lead to an
   intermediate artificial empty component, as long as the user has
   not finished typing. For example the user types the words "foo bar":

   (cdr (orderless-compile "foo")) => ("foo")
   (cdr (orderless-compile "foo ")) => ("foo")
   (cdr (orderless-compile "foo bar")) => ("foo" "bar")

Orderless expects a list of functions but a hook is not simply that. A hook can
also be a single function, or a list which includes the special value t to
reference the default value of the hook.

By not only checking the compiled regexp, smart case is also toggled when
matching on annotations, e.g., &Lv as in #184.