Change "settings['vue-i18n'].localeDir" to can specify object options.

[ota-meshi]

This PR changes settings['vue-i18n'].localeDir to allow object option.

fixes #10

The option allows the user to specify the following options in the object:

• pattern (string) ... A glob for specifying files that store localization messages of project.
• localeKey ('file' | 'key') ... Specifies how to determine the locale for localization messages.
  - 'file' ... Determine the locale name from the filename. The resource file should only contain messages for that locale. Use this option if you use vue-cli-plugin-i18n. This option is also used when String option is specified.
  - 'key' ... Determine the locale name from the root key name of the file contents. The value of that key should only contain messages for that locale. Used when the resource file is in the format given to the messages option of the VueI18n constructor option.

I considered how to let the user specify the depth of the hierarchy instead of the localeKey, but I realized that when I specified the depth of the hierarchy, the locale could not be specified.
I thought perhaps the user would either use vue-cli-plugin or give it directly to the VueI18n constructor and use it.
So I decided to provide two options, file and key.

The other change was to show in the message of the no-missing-keys rule which locale is missing.

[kazupon]

Perfect! 💯
Thank you so much!