Add a “Download them all” subcommand

[bfontaine]

Proposed by Reddit user neoice. In addition, we could sync the RFCs that have changed since the last download/sync.

There are two possibilities:

• Using rsync: Easy to synchronize, but we would have to change the filenames of the RFCs (actually, we're using $rfc_dir/N, where N is the RFC number) to match the one used by rfc-editor.org (rfcN.txt).
• Download the weekly .tar.gz provided by rfc-editor.org: Harder to synchronize with (even if rfc-editor.org provide a weekly .tar.gz which contains all RFCs which changed during the past 7 and 30 days), since we would have to remember the last time we synchronized.

Update:

regarding the second possibility:

• rfc sync all: download all RFCs
• rfc sync month: 30 days
• rfc sync week: 7 days