Add quotes to paths in MeCab arguments
Without quotes directories with spaces in them will fail to be processed
correctly.

This bug was first reported in the fugashi repo [here](https://github.com/polm/fugashi/issues/24). It's not a bug in fugashi - fugashi handles quoted paths correctly, and the relevant dictionary packages quote paths in `MECAB_ARGS` to handle this case. Looks like it was missed when the dictionary handling code was added, since it doesn't use `MECAB_ARGS` but instead builds args up from component parts.