IndexFieldList now can emit multiple fields

Example:

    $ taskdo IndexFieldList --index bsz --doc-type title \
                            --fields "content.001 content.245.a 245.b" \
                            --null-value "UNDEF"

IndexFieldList is for the moment the elasticsearch counterpart
to [marctotsv](https://github.com/miku/gomarckit)
v0.1.59