Add support for matching all section labels

[GollapudiSrikanth]

This adds support for configuring how section labels are matched when generating changelog sections.

Previously, a section matched an issue when any configured label matched one of the issue’s labels. This made it difficult to define sections that require a combination of labels, such as breaking change and library: mobile.

This change adds a new label-match section property with two options:

• any: existing behavior and the default
• all: requires all configured labels to match

Closes #79

Example:

changelog:
  sections:
  - title: "Mobile Breaking Changes"
    labels: ["breaking change", "library: mobile"]
    label-match: all