sources: remove references to ads and dcc

which are unfortunately currently not actually supported.
update/fix up README
Provide appropriate error message for nci if python-urwid not installed.
add function to test for and import the nci module.
nci: use xdg-open for external handler calls

This introduces a dependency on xdg-utils, but it should provide more
consistent support.
cli: remove missed references to ads and dcc sources
use xdg-open in xapers-importer
update man page on view command
update readme
update color palette
cli: handle non-existant input file
clean up xapers-importer
test: have tests clean up after itself
tweak the readme about debian-snapshot
nci: tweak colors
nci: add arrow keys for navigation