Updated the Html2Bbcode class.

Renamed the parse method to convert.
Updated spec for the Html2Bbcode class.

Renamed the let - sample_html to input_html.
Updated the html2bbcode class.

Covered the b bbcode tag.
Updated the html2bbcode class.

Covered the i bbcode tag.
Updated the html2bbcode class.

Covered the u bbcode tag.
Updated the html2bbcode class.

Covered the s bbcode tag.
Updated the html2bbcode class.

Refactored the #convert method.
Extracted html tags & special html tags into separate methods.