User can now mark place in list_format from where he wants to start
right-aligning remaining content. Tab character (\t) is used as marker.

Added also 'top' alias example to topydo.conf which utilizes this
feature and implements #52.

Minor:
    - remove `- 1` from statements setting line width
    - move strip_braces() to topydo.lib.ListFormat and rename it to
      strip_placeholder_braces()

fixup strip_braces