mkv: get metadata from info element

[dedobbin]

As discussed here, #273
I just placed the segment title as a tag.
When dumping the metadata for an mka file that has both a "tag title" and a "segment title" it looks like this

Metadata {
    revisions: [
        MetadataRevision {
            tags: [
                Tag {
                    std_key: None,
                    key: "TITLE",
                    value: String(
                        "real_title",
                    ),
                },
            ],
            visuals: [],
            vendor_data: [],
        },
        MetadataRevision {
            tags: [
                Tag {
                    std_key: None,
                    key: "TITLE",
                    value: String(
                        "tag_title",
                    ),
                },
...

If you think there is a neater way let me know!