[FUNC] Add an option for truncation from left

[wonjininfo]

Minor feature update, as discussed in the Friday meeting.

When the option --truncation_side_left is selected:

• For an input sequence shorter than max_len, this option will not affect it. Padding will still be attached at the end of the example.
• For an input sequence longer than max_len, the beginning part of the sequence will be deleted to fit within the max_len restriction.

[tmills]

@wonjininfo if you can double check my change to make sure it keeps the intended behavior then i'll merge the branch

[wonjininfo]

Looks good to me!