Add access to more Node fields

[polm]

From #75:

C. Could you also provide wrapper functions for other fields in mecab_node_t as well?
https://taku910.github.io/mecab/doxygen/structmecab__node__t.html
some of the attributes like right attribute id, left attribute id, word cost, best accumulative cost from bos node to this node, isbest, etc
are useful to figure out how the cost varies in a sentence, and maybe useful for adjusting the cost in user dic

I am not actively working on this, PRs are welcome. It should not be a big change. It will cause binary incompatibility if there are serialized Nodes for some reason, which I would recommend against.