Add styles 'italic' and 'strikethrough' and test.

[dclause]

Adds the missing styles 'italic' and 'strikethrough'.

This closes #25

It also duplicates the PR #26 which is stated out-of-date by the maintainer. I am very sorry for I do not know how to get on board of a PR that is not mine. I which this one could credit PR #26 author too.

Compared to #26 this PR also:

• add a test for the two new styles
• add .italic() (the most widely supported style) in the basic_examples, and hence in the readme

It does not: expands the shell "support" matrix at the end of the readme since I could not get that info.