Add command to subset seqrepo

[holtgrewe]

Is your feature request related to a problem? Please describe.
The whole seqrepo is of size 216GB.

Describe the solution you'd like
Implement a CLI tool that copies over the data for transcripts in cdot files (after #42) plus the one for the genomes into a fresh seqrepo.

Describe alternatives you've considered
N/A

Additional context

• Record-wise storage of JSON in sqlite databases #42