Add flag to include issue milestone in csv output

[mancdaz]

Add a flag to include an extra column in output, heading is 'Milestone' and the content would be the milestone name for the issue (each issue can only have a single milestone, unlike labels). This allows easy filtering on milestones in the agile tool.

[git-harry]

@mancdaz once the conflicts are fixed I'll test this out.

[mancdaz]

@git-harry thanks...all fixed now

[mancdaz]

@git-harry moved the logic into the field_to_callable method, and made inclusion of milestones part of default behaviour, as discussed

[git-harry]

@mancdaz I've tested this and it seems to work fine. Assuming it still works with the agile tool I think this is good to go.

[mancdaz]

Thanks @git-harry