[SPARK-10299][ML] word2vec should allow users to specify the window size

Currently word2vec has the window hard coded at 5, some users may want different sizes (for example if using on n-gram input or similar). User request comes from http://stackoverflow.com/questions/32231975/spark-word2vec-window-size .

Author: Holden Karau <holden@us.ibm.com>
Author: Holden Karau <holden@pigscanfly.ca>

Closes #8513 from holdenk/SPARK-10299-word2vec-should-allow-users-to-specify-the-window-size.