If two headers have similar texts, the generated id could be equal. In order to prevent id clash:
- A unique suffix is added if a header id already exists
- Option to add a prefix to header id
- Update of correspondent tests
- (credits to nicovalencia)
Closes#81, closes#82
When the text is pulled from indented HTML elements, ex:
```
<body>
<div>
## Content to be converted
</div>
</body>
```
it no longer becomes wrapped in code/pre tags. A new option is also available (allowBlockIndents) that when set to false, reverts to the previous behavior
