Lack of functionality to export narrowPeaks files

[PeteHaitch]

I noticed this while reviewing Bioconductor/Contributions#2568
The lack of an export method for narrowPeaks files meant that the authors opted to roll their own, before eventually moving to plyranges:write_narrowpeaks().

Should the plyranges:write_narrowpeaks() functionality be migrated to rtracklayer or BiocIO (I'm not really sure of the intended relationship between these 2 package) and it become a wrapper (like the other plyranges::write_*() functions seem to be).
Tagging @sa-lee who might know some background or further context on the issue?