Modify Indexed Characters limit
I think that I could define in the document itself a value for `_indexed_chars` as an optional setting for the FSRiver:

* `indexed_chars : 0` (default) will use default mapper attachment settings (`index.mapping.attachment.indexed_chars`)
* `indexed_chars : x` will compute file size, multiply it with x and pass it to Tika using `_indexed_chars` field.

That means that a value of 0.8 will extract 20% less characters than the file size. A value of 1.5 will extract 50% more characters than the filesize (think compressed files).
A value of 1, will extract exactly the filesize.
Closes #17