Fixed MD links attached to the right span instead of the whole sentence

[DiazBejaranoD]

Fixes #97

Expanded resolve_links feature to allow the correct span of text to be used in the MD link.
In v0.0.17, the whole sentence is used as the link text.

Limitations:

• It still does not support multi-line links.
• It still does not support links within headers.

[yumingmin88]

Hello, using your method found that char_index_max causes the array to cross bounds, and char_index_max keeps decreasing <0 until it exceeds the text length
Using avg_char_width to calculate the beginning and end of links is inaccurate and does not reflect the exact width of each character