Initializing new tag if none present

Closes https://github.com/watermelonexpress/benchprep-tagster/issues/2

Ensures that default tag is 0.0.1_SHA for minor and 0.1.0_SHA for major builds

```ruby
➜  be tag --dry-run
** DRY RUN **
Checking the git repo - ✓
Fetching latest tags
Finding latest remote tag - fatal: No names found, cannot describe anything.
0.0.0_0Generating new tag - 0.0.1_f5f69
Not creating tag 0.0.1_f5f69
Not pushing tag 0.0.1_f5f69
** COMPLETE **


➜  be tag --dry-run --major
** DRY RUN **
Checking the git repo - ✓
Fetching latest tags
Finding latest remote tag - fatal: No names found, cannot describe anything.
0.0.0_0Generating new tag - 0.1.0_f5f69
Not creating tag 0.1.0_f5f69
Not pushing tag 0.1.0_f5f69
** COMPLETE **
```