Add mark_read option and fix #45

[pemontto]

This PR adds an option mark_read set by default to true to maintain backwards compatibility. It truly allows users or other services to use the mailbox simultaneously with the IMAP input plugin.

Changes:

• Add mark_read option
• Grab all messages if uid_tracking is true and uid_last_value is nil
• Add more debug messages for troubleshooting
• Add some more detail to the docs

Fixes #45

[pemontto]

Fixed merge conflict

[maxhodak]

Is there a reason this hasn't been landed? It seems like it would be very useful.

[pemontto]

Any chance of a review/merge @robbavey @karenzone

[AlbertasB]

Would really like to see mark_read functionality merged

[spacemoose]

I would also like to see this merged.

[pemontto]

Maybe @mashhurs can take a look?

[jsvd]

@logstashmachine bump minor

[jsvd]

I don't have a good way to test this right now, @spacemoose and @pemontto can you confirm this works as expected?

[karenzone]

Thanks for the docs changes. I left some suggestions for your consideration.