initial step of flag for specifying tarsum Version

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>
versions: checking sums of versioned tarsums

this commit allows for basic validation of checksums that include
versioning. Though it is still dependent on the flags for it to match
calculated incoming tar, to the versions in the check file.

```
vbatts@noyee ~/src/vb/docker-utils (master *) $ docker save busybox | ./dockertarsum -t 1 > b.s
vbatts@noyee ~/src/vb/docker-utils (master *) $ docker save busybox | ./dockertarsum -t 1 -c ./b.s
511136ea3c5a64f264b78b5433614aec563103b4d4702f3ba7d4d2698e22c158:  OK
df7546f9f060a2268024c8a230d8639878585defcc1bc6f79d2728a13957871b:  OK
e433a6c5b276a31aa38bf6eaba9cd1cfd69ea33f706ed72b3f20bafde5cd8644:  OK
e72ac664f4f0c6a061ac4ef332557a70d69b0c624b6add35f1c181ff7fff2287:  OK
vbatts@noyee ~/src/vb/docker-utils (master *) $ docker save busybox | ./dockertarsum -c ./b.s
511136ea3c5a64f264b78b5433614aec563103b4d4702f3ba7d4d2698e22c158: FAILED
df7546f9f060a2268024c8a230d8639878585defcc1bc6f79d2728a13957871b: FAILED
e433a6c5b276a31aa38bf6eaba9cd1cfd69ea33f706ed72b3f20bafde5cd8644: FAILED
e72ac664f4f0c6a061ac4ef332557a70d69b0c624b6add35f1c181ff7fff2287: FAILED
./dockertarsum: WARNING: 4 computed checksums did NOT match
```

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>