Github Flavored Markdown does not parse underscores in the middle of a word as emphasis/bold.
This commit adds this feature to showdown through an option called "literalMidWordUnderscores".
Related to #164
Github Flavored Markdown detects urls and mails embeded in the text without any extra markup or delimiter.
This commit adds this feature to showdown through an option called "simplifiedAutoLink".
Related to #164
This feature allows users to define the image dimensions using markdown syntax:
```
```
To enable this feature, use the option `parseImgDimensions`.
Closes#143
