Markdown filter: extra newline inserted on merge after subfiltering yaml header

[Chase Tingley]

Weird bug. If you enable translation of header metadata, the filter appends an extra newline at the end of the header on merge.

To reproduce, use the attached files:

tikal.sh -fc [e-mail] -x test-yaml-header.md 
tikal.sh -fc [e-mail] -m test-yaml-header.md.xlf 

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[Chase Tingley]

Fixed by #549