Fixes inefficient updating N-gram counts.
NOTE: Using '--binary' option (this option is not enabled by default yet)
for saving outputs would lead to significant speed up.
By replacing std::map with boost::unordered_map.
Runtime of extractor on 100-best lists of 2679 sentences:
Before:
real 0m35.314s
user 0m34.030s
sys 0m1.280s
Ater:
real 0m26.729s
user 0m25.420s
sys 0m1.310s