add option to set criteria which type of emails should be requested

[CordlessWool]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

In the old version of "email in" node only unseen version are requested. With this changes, a user could select what type of criteria he/she want to set (all, seen, answered, flagged, unanswered, unflagged, unseen).
Get seen messages, forum request

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the mailing list/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage increased (+0.04%) to 65.821% when pulling 167e21d on CordlessWool:master into 8178153 on node-red:master.

[dceejay]

Great - thanks for this update.