add media/page archiving capabilities

[nodiscc]

Moved from #22

add a subcommand parser dedicated to data archival:

• should be implemented in a library form, with a console entrypoint that may serve as a Minimal Working Example

these operations should be performed separately:
query a Shaarli instance to get a list of links
parse a list of links and retrieve/archive corresponding media

Archival preferences could be specified in a config file

• local archive directories
• multimedia preferences, e.g. audio & video formats

Suggestions:

• move API commands to an api subparser, and declare other subparsers for specific actions:

   $ shaarli api <params>
   $ shaarli archive <params>
   $ shaarli <action> <params>

there's also the possibility of providing an interactive CLI entrypoint using the click library (possibly overkill but potentially quite fun to write :) )

[nodiscc]

Closing, see #41.