Extend MeCab plug-in to support morpheme n-gram

[kmaehashi]

Currently MeCab plugin just splits the input string and use surface of each morpheme as a feature.
It is better to support morpheme n-gram to extract the context of the natural language.

I'm thinking of adding a new "ngram" parameter (which is optional and defaults to 1) to the mecab plug-in configuration as follows:

"string_types": {
  "mecab": {
    "method": "dynamic",
    "path": "libmecab_splitter.so",
    "function": "create",
    "arg": "-d /usr/lib64/mecab/dic/ipadic",
    "ngram": 2
  }
}

For instance, when 本日は晴天 is given as an input, {"本日", "は", "晴天"} is extracted when "ngram" is 1, whereas {"本日|は", "は|晴天"} is extracted when "ngram" is 2.

[kmaehashi]

Updated docs for this: jubatus/website#236

[kmaehashi]

Fixed via #1070