Github Flavored Markdown supports strikethrough (`<del>`) syntax using double tilde `~~` delimiters.
This commit adds this feature to showdown through an option called "strikethrough".
Related to #164
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
Old extensions that register themselves in `showdown.extensions` can be loaded and validated using the new extension loading mechanism.
However, a warn is issued, alerting users and developers that the extension should be updated to use the new mechanism
BREAKING CHANGE: Deprecates `showdown.extensions` property. To migrate, you should use the new method `showdown.extension(<ext name>, <extension>)` to register the extension.
