GitLab issues as a storage provider

**Is your feature request related to a problem? Please describe.**
Have you given any thoughts to allowing additional storage providers? I've been wanting to build a similar CLI task management tool, backed by GitLab issues using the API. But now I'm thinking...what if we expand taskbook to allow for N number of storage providers.

**Describe the solution you'd like**
Decouple storage from the file system, allowing anyone to write a shim between the taskbook data model and an external storage provider (think GitLab, GitHub issues or Trello/JIRA or S3, etc.)

**Additional context**
Spoiler alert: I work for GitLab 😄 🦊

The issue names no files, tests, or entry points. Start by locating taskbook's data model and filesystem storage implementation, then determine how a GitLab issues provider could fit the proposed provider boundary. Done criteria and the provider interface are not specified and would need agreement before implementation.

Written by the indexing model from the issue text.