Add `!else` support for conditional block

Add `!else` support for conditional block

This patch adds the support of `!else` within an `!if` context:

    !if something
      11 22 33
    !else
      44 55 66
    !end

This is just equivalent to:

    !if something
      11 22 33
    !end

    !if {not something}
      44 55 66
    !end

but more readable/natural.

Change-Id: I0efa68a65c485d7dbf8124cd51fc5ed3c1ac9f46
Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>