Add posibility to define model_dir as an array of directorie paths

Sometimes in projects you have shared modules that contains shared models.
This patch adds the possibility to define multiple dirs in the option `--model-dir`.
It also fixes a bug (only present in version 2.6.5, not master): in case of defining options in the rake task `:set_annotation_options`, `:model_dir` and  `:ignore_columns` options (`OTHER_OPTIONS` array) was not taking into account.