Enable removal of default/added tag/label from issues in IMYML

[briandominick]

In an issue's tags Array, any item string prefixed with a hyphen (-) character will removed from the default or appended tags list.

Example:

$meta:
  proj: myorg/myrepo
  defaults:
    tags: [+posted-by-issuer,needs:label,+needs:docs]
issues:
  - summ: Fix misstatement in the README
    body: |
      Correct that really bad typo on the first paragraph.
      No docs needed for this change since it's a docs change.
    tags: [documentation, -needs:docs]
  - summ: Issue that will be fixed immediately
    tags: 
      - '-needs:label'

Release Note

Users may now append tags to existing tags lists by establishing them in the $meta.defaults.tags Array, prepended with the + character.

[briandominick]

Resolved by PR #24