Add tags support

[asmfreak]

Rewording of #12 and mention of #15
We need tags support in habitipy

[hardaker]

I've started tags support in github.com:hardaker/habitipy and will eventually submit a PR for it (likely after other PRs are merged to avoid conflicts).

example: git todos add -t tag1,tag2 my new task

git todos

1. my new task (tag1, tag2)