[examples/seq2seq] defensive programming + expand/correct README
This PR deals with the new s2s script and its usage - mostly documentation.

This PR:

`run_seq2seq.py`:
* checks for invalid column names

`README.md`:
* largely expands the document explaining and exemplifying the supported formats
* documents the nuances of t5 and mbart translation - I hope we fix this on the programmatical level in the future 
* fixes examples where scores were bad - all examples were verified to work and provide good scores, including the custom files, which were far from easy to figure out. Hopefully now it'll be easier.
* makes the examples quick to complete by running only a short sample - this is important to notice breakages, e.g. in eval stage - nobody is going to wait for train to complete in hours.
* adds cnn/daily mail dataset
* recovers one preprocessed dataset from the last s2s incarnation recommendation: it is offered for high bleu scores (the other 3 are either identical or are just slightly worse than the preprocessed ones - full porting status: https://github.com/huggingface/transformers/issues/10044)

@patil-suraj, @sgugger 