add scratch util ala p to create temporary dirs

i got tired of myself leaving throwaway stuff all over my disk
so i wrote this little tool to help me manage that.

directories under ~/Scratch should never contain anything that needs to
be persisted. they have to be assumed to be deleted as soon as i'm done
with them.

it works as following:

$ scratch

    will cd to ~/Scratch

$ scratch foo

    - if a folder named foo exist, it will go to that.
    - if one or more folders with foo in the name exists, it will cd to
      the one with the shortest name.
    - if no folder with a name matching foo exists, it will create one
      named foo and cd to it.

i could just use /tmp/ for this, but i wanted something where i could
just rm -fr at will without thinking about it.