Fix rendering in indented blocks

It fixes issue #31 with rendering indented fenced blocks. They were rendered only when put at beginning of the line. To illustrate, this block was processed correctly

```plantuml
A --> B : I am processed
```

while a block nested under a list item (indented) was not processed

  * A list item with nested block

    ```plantuml
    A --> B : I am not processed
    ```

With this patch the block is converted into image and correctly put in the document tree allowing for images nested in other block elements, e.g. list items.

mikitex70 / plantuml-markdown

Fix rendering in indented blocks #32