MSWrite plugin default codepage issue

This is the patch I propose to deal with the default codepage issue.

It follows a non-working version committed earlier and allows the user to
select his/her favorite default codepage by passing an importer argument,
namely '-i mswrite-codepage:CP1250' (CP1250 in this example, but every other
supported encoding allowed).

If you are effected by this issue, please test whether this solution works
for you. I'll be away a few days and will join discussion when I'm back.

Ingo