Make a SlackwarePackage have an optional internal URL
Bump version number
Build the remote list right after an "update" operation
Fix update-manifest operation

Previously, the update-manifest operation relied on
/var/slackroll/FILELIST.TXT being present when it was going to be run.
From that file, the program extracted the list of MANIFEST.bz2 files it
had to download and process.

Since the "update" operation now processes FILELIST.TXT just after
downloading it and then deletes it, the file is not available at
"update-manifest". The solution is to extract the list of MANIFEST.bz2
files when processing FILELIST.TXT during the "update" operation.

This list is then stored together with the mirror URL on disk in a file
called manifestlist.db that can be loaded with try_load. The data
structure has this format:

    [ (mirror_url, file_path), (mirror_url2, file_path2), ...]

The "update-manifest" operation now needs to load the list of
MANIFEST.bz2 files from manifestlist.db and proceed to download and
process them as it did before.
Properly indent op_num_args dictionary
Add add-repo, list-repos and remove-repo operations

Note external repositories are not used anywhere yet. This merely adds
operations to manipulate the list of external repository URLs.
Minor improvements to "add-repo" documentation
Add support for handling external repository packages
Fix repository package downloads by using the right URL
Update slack-desc description and include the project URL
Update Github pages URL in manpage and help files
Split "update" operation warnings and actively check for transient packages
Fix downloading of info files from external repos
Fix package signature URLs in URL listing
Mention repository operations in full and brief operations list