Tabs should be expanded based on their position in the line

This pull-request addresses issue vfmd/vfmd-spec#2

Specification:
  - Expands tab characters based on their position rather than always expanding them to 4 spaces

Syntax guide:
  - Refrains from using the tab terminology because actually, the implementation is unaware of tabs. 

    For example:

    ```
    >[TAB]something
    ```
    will not be recognized as codeblock within a blockquote as per the specification. Therefore, we should not say in the userguide that users can get a code-block by starting the line with either 4 spaces or 1 tab. Only 4 spaces will work consistently.

  - Explains about how tab characters are handled in a note at the start of the guide