Add CSAF downloader

* Dense and refactor ROLIE code in aggregator a bit.
* Move  advisory file processor to csaf package.
* Fix minor typo on main readme

docs/csaf_downloader.md

@@ -0,0 +1,18 @@
+## csaf_uploader
+
+### Usage
+
+```
+Usage:
+  csaf_downloader [OPTIONS] domain...
+
+Application Options:
+  -d, --directory= Directory to store the downloaded files in
+      --insecure   Do not check TLS certificates from provider
+      --version    Display version of the binary
+  -v, --verbose    Verbose output
+  -r, --rate=      The average upper limit of https operations per second
+
+Help Options:
+  -h, --help       Show this help message
+```