add ability to get issues from multiple repos

[mancdaz]

• add flag to pass in multiple repo names
• return all labels for all repos and use for headers
• write headers once
• process each repo in turn and write out issues

[mancdaz]

Thanks for the review @git-harry - changes made and ready for review again

[mancdaz]

ok @git-harry I've made it so that we only get, and pass around, a set of label names. The appropriate adjustments have been made in the places where a list of label objects were expected.

[git-harry]

Looks to work okay. I gave the output a quick test in the graphing tool and it didn't seem to be affected by the duplicate IDs so I think this is ready.

[mancdaz]

Thanks @git-harry