Slug string from URI-unsafe characters

[kogai]

- Do you want to request a feature or report a bug?

FEATURE REQUEST
(Maybe, related to #414)

- What is the current behavior?
In case of using editorial workflow and GitHub backend.

When we create entry with URI-unsafe characters in title like 日本語のタイトル or عنوان فارسی, we can't create PR correctly (It is created with the title of empty string)

- What is the expected behavior?

If we want to create entry with URI-unsafe characters, we only do that slugify URI-safe characters.
As below

slugFn("日本語のタイトル") // expect "日本語のタイトル"
slugFn("日本語 Abc タイトル") // expect "日本語-abc-タイトル"

- Please mention your node.js, and operating system version.

v8.2.1

[erquhart]

Should have been addressed by #414, please re-open if still an issue.