Added italics and bold styling markdown syntax features

[gulyapulya]

Fixes #23

My intention was to implement additional syntax features for the markdown files such as italics and bold. They should be transformed into html properly and right now they are not.

My solution is to replace all markdown styling symbols with the corresponding html elements. In markdown, *sample text* or _sample text_ are used to display the sample text in italics and **sample text** or __sample text__ are used to display the sample text in bold. Therefore, for html, they have to be transformed into <i></i> and <b></b> elements respectively. I integrated my code into the existing markdown module. I combined it with the links replacement part as it uses the same technique.

I also added new lines to the testing markdown files to check my features.

[gulyapulya]

@rokaicker Could you please review my pull request?

[rokaicker]

Hi @gulyapulya, the changes look good!

I was wondering, could you add a section in the "Overview" section documenting the capabilities of the markdown parser? (like maybe say the program is able to parse markdown files and properly parse, italicized, bolded, heading text, etc.)?

[gulyapulya]

@rokaicker, yes, sure!

The overview now contains information about the supported markdown styling features.