Updated the qgram tokenizer to pad the input strings.
Specifically, added three parameters to qgram tokenizer constructor: padding, prefix_pad, suffix_pad. These parameters control padding and what characters that should be used for padding.
Coverage remained the same at 100.0% when pulling 77f0eb539abd429922b0470708a2bbae72611349 on enhan_update_qgram into 02d73ec20dc3b16fbeaf8bd4edb5fb69d9f21a8b on master.
Coverage remained the same at 100.0% when pulling 7619caf462ff5e664b5f6b6b6e13b548fda437f4 on enhan_update_qgram into 02d73ec20dc3b16fbeaf8bd4edb5fb69d9f21a8b on master.