fixed incorrect titleizing of unicode string

[sio4]

What is being done in this PR?

• fixed incorrect Titleize() of Unicode string
• it fixes incorrect Humanize() which is now depends on Titleize()

fixes #67

What are the main choices made to get to this solution?

The previous code uses rune(part[0]) to pick the first character of the string but this is incorrect usage of the rune type. It should be []rune(part)[0]. We can also consider using golang.org/x/text module to handle this issue better in the future.

What was discovered while working on it? (Optional)

Titleize() ignores a part that starts with a special character such as `, * so the result will be:

• "using rune correctly" --> "Using rune Correctly" (which is fine)
• "what a wonderful world" --> "What A wonderful World" (which is odd to me)

We need to reconsider the design.