Add constructor to Conll for creating new one from list of sentences

[matgrioni]

Conll objects should be able to be constructed from an existing sequence of sentences. It is not always possible to have a string iterable to recreated it, and this also facilitates better manipulation of groups of sentences (for example splitting into dev, train, and test is not possible right now).

With my hope for pyconll to eventually read sentences of multiple formats while keeping good typing hints this may not be easily possible to achieve both, but should be used to choose between multiple options.