Adds a --label-group argument that filters exported issues and pull requests by label using Disjunctive Normal Form (DNF) logic — OR of ANDs (excluding NOT).

Each --label-group flag accepts a comma-separated list of labels that must all be present on an issue (AND logic). Specifying multiple --label-group flags matches issues satisfying any of those groups (OR logic). Label matching is case-insensitive.

Examples:

# Issues with BOTH labels (AND):
gh2md owner/repo out.md --label-group "bug,help wanted"

# Issues with EITHER label (OR):
gh2md owner/repo out.md --label-group "bug" --label-group "help wanted"

# (bug AND v1.0) OR (bug AND v2.0):
gh2md owner/repo out.md --label-group "bug,v1.0" --label-group "bug,v2.0"

When no --label-group is specified, all issues/PRs are exported (existing behavior preserved).