Fixed the invalid type error while running init with a numeric folder name

[sumimakito]

This PR fixes an invalid type error that occurs while running hexo init with a numeric folder name.

E.g. Executing hexo init 233 will produce the error as follows:

FATAL Something's wrong. Maybe you can find the solution here: http://hexo.io/docs/troubleshooting.html
TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type string. Received type number
    at assertPath (path.js:39:11)
    at Object.resolve (path.js:1088:7)
    ... (Omitted)

Since there'll be a need to create a new hexo project with numeric folder name in some cases, I've ensured that a string will be passed to the resolve() function.

[coveralls]

Coverage increased (+0.4%) to 86.071% when pulling e7fc5b9 on SumiMakito:fix_using_number_as_init_folder_name into bf6a072 on hexojs:master.

[coveralls]

Coverage increased (+0.4%) to 86.071% when pulling e7fc5b9 on SumiMakito:fix_using_number_as_init_folder_name into bf6a072 on hexojs:master.

[yoshinorin]

LGTM! Thanks!
This change works well.