Text without style should be processed as style of type "unstyled"

[SamyPesse]

Currently, inlineStyleRanges does not contain range for unstyled text, it's causing the markdown syntax to be escaped correctly.

The workflow for applying inlineStyleRanges should be:

• Linearize ranges
• Fill empty ranges (text without ranges) with { offset: X, length: N, style: 'unstyled' }

Since the method is called recursively, text inside bold/italic/... will be correctly escaped when required.