Add support for news-to-metainfo

[somas95]

Appstreamcli recently gained support to converting NEWS files to appstream-metadata/metainfo ones. While currently appstream-utils can do something like that, appstreamcli has some advantages:

• You can limit the number of releases to convert
• You can merge the resulting xml in the existing metainfo file, so you can have a workflow like this:

+------+  +-------------------+
| NEWS |  | *.metainfo.xml.in |(localized, without releases info)
+--+---+  +---------+---------+
   |                |
   +----------------+
   v
 appstreamcli news-to-metainfo --limit=3
   +
   |
   v
+--+--------------+
| *.metainfo.xml  | (with the las 3 releases info)
+-----------------+

The relevant code is in https://github.com/ximion/appstream/blob/master/src/as-news-convert.c, sadly I'm not proficient enough in C to make a MR