reader: add method to get Primary Volume label

[codecov[bot]]

Codecov Report

Patch coverage: 83.33% and project coverage change: +0.02 :tada:

Comparison is base (30ab35f) 78.63% compared to head (7f00357) 78.66%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #35 +/- ## ========================================== + Coverage 78.63% 78.66% +0.02% ========================================== Files 6 6 Lines 1161 1167 +6 ========================================== + Hits 913 918 +5 - Misses 175 176 +1 Partials 73 73 ``` | [Impacted Files](https://app.codecov.io/gh/kdomanski/iso9660/pull/35?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Kamil+Doma%C5%84ski) | Coverage Δ | | |---|---|---| | [image\_reader.go](https://app.codecov.io/gh/kdomanski/iso9660/pull/35?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Kamil+Doma%C5%84ski#diff-aW1hZ2VfcmVhZGVyLmdv) | `76.58% <83.33%> (+0.26%)` | :arrow_up: |

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