Add missing params ddoc to parser factory function

[joseph-wakeling-frequenz]

The handler parameter was presumably omitted as optional, but this causes docs/ddox builds to fail (including for downstreams that have std.experimental.xml as a dependency).

[joseph-wakeling-frequenz]

Note, this issue might have been caught if building docs was part of the project's CI. Are there any preferences for how this should be done (other than the trivial one of adding a make docs element to the script: array in Travis config)?

[rikkimax]

@wilzbach you appear to be the current maintainer based upon code history, do you have any preference or are happy to go with anything that is sane?

[wilzbach]

you appear to be the current maintainer based upon code history

I think I just saved this package from oblivion.

do you have any preference or are happy to go with anything that is sane?

Anything that's sane. Thanks!

[joseph-wakeling-frequenz]

OK, I'll push a follow-up patch to this PR in a couple of minutes. Thanks!

[joseph-wakeling-frequenz]

Here you go.