Added support for dict seq_len for different lengths of sources in text supplier

[noajshu]

usage (e.g., for sequences named "review" and "sentiment"):

  data:
    - text:
      seq_len:
        review: "{{ 50 }}"
        sentiment: "{{ 1 }}"
    # other opts...

The old format still works, specifying a single seq_len for all sources:

  data:
    - text:
      seq_len: 100
    # other opts...