Allow keypaths that contain hyphens with numbers such as '404-title'

@cjoudrey The keypath regex wasn't working for `"404-page"` because the hyphen wasn't supported when prefixed with numbers. I've fixed that in this commit. Please review and comment. :)

**before**
`my-page` worked
`404_page` worked
`404-page` didn't work

**after**
`my-page` worked
`404_page` worked
`404-page` **now works**

/cc @nickjs