Support for line annotations

[DBAnthony]

Adds support for line annotations from config (to e.g. separate out categories, mark out important value, etc)

Example usage:

annotations:
        - vlines:
              values: [600]
              axes: ['main','summary']
              style: '--'
              label: 'Overflow bin for fit'
        - vlines:
              values: [200]
              colour: 'red'
        - hlines:
              values: [0.75, 1.25]
              style: '--'
              axes: ['summary']