allow setting tiff SoftwareTag, DocumentNameTag, ImageDescTag

[jdries]

There's a number of tiff tags that currently can not be written to geotiff in Geotrellis.
For instance: SoftwareTag, DocumentNameTag, ImageDescTag
These are already defined here:

geotrellis/raster/src/main/scala/geotrellis/raster/io/geotiff/tags/codes/TagCodes.scala

Line 33 in fa88aed

val ImageDescTag = 270

The request is to either add explicit support for these tags, or to allow a more generic way to customize the tags that are written so that we can add any tag.