There is no easy way to format text inside text object. With some crazy math skills, you could create multiple text objects for each differently formatted string within the text.
Alternatively, you could use the HTML text plugin which name I fail to remember; you give it HTML code and it shows the formatted text. However, it has its limitations, such as being rendered above canvas and thus not influenced by shaders/layers.
Finally, there is spritefont, which again has its limitations, specifically the fixed width. You could have different styles - regular, italic, bold, italic bold. But you probably want to use regular fonts that aren't monospaced...
Here, check this plugins list for plugins that you can use to this effect. Try Pode's WebFonts or JesseJoh's HTML elements.