Added possibility to handle files with duplicate keys

[IcePlanet]

This change adds possibility to handle files with duplicated keys.
It is fully backwards compatible and default bahavior is the same as previous version. On top of this it allows by configuration to read files with duplicates and also to store either 1st or last key value.

Background for the change: regardless if this is correct or not, sometimes you need to read config file where you have 0 influence on content, in that case it might be handy to allow read duplicates.

Creating list with all duplicates is NOT supported in this change and also duplicate sections are NOT supported in this change.

[coveralls]

Coverage remained the same at 81.12% when pulling 76d4171 on IcePlanet:master into 7a0f1e0 on DiffSK:master.

[david-fliguer-ws]

Hi, has this been merged or not yet ?

[joesanford]

Are there any updates on when this may get merged? I have a good use case for this functionality (handling ini-formatted payloads that can error and kick out duplicate keys), and would like to remain in stream with changes you make. Otherwise, I may fork with this change.

[jelmer]

This appears conflicted.

Also, shouldn't this have an option to preserve all values?

Could we just allow passing in strings for the duplicate_keys enum? The constants are quite long and inconvenient.

Please also add some tests.