Make embeddings optionals

[lfoppiano]

Preliminary test on the date model.

• with embeddings:

                  precision    recall  f1-score   support

           <day>     0.9362    0.9362    0.9362        47
         <month>     0.9508    0.9667    0.9587        60
          <year>     0.9841    0.9688    0.9764        64

all (micro avg.)     0.9591    0.9591    0.9591       171

• without embeddings:

                  precision    recall  f1-score   support

           <day>     0.9388    0.9787    0.9583        47
         <month>     1.0000    0.9333    0.9655        60
          <year>     0.9403    0.9844    0.9618        64

all (micro avg.)     0.9593    0.9649    0.9621       171

We need to average 5 runs, though, but the results are comparable which convinced me that I could create this PR.