Allow specifying a column type for each layer

Closes #93
Includes testcases
Produces Mapnik XML using the "pgraster" input driver:
https://github.com/strk/mapnik.plugin.input.pgraster
Accept "extra_ds_opts" for per-layer extra datasource options
Merge tag '0.20.0' into pgraster

Conflicts:
	NEWS.md
Merge branch 'master' into pgraster-merge

Conflicts:
	NEWS.md
pgraster columns types changes proposal
Better documentation for the changes in gcols param/option about the default types.
Moves new changes to the proper version. Next version will be 0.21.0