Striprtf

* Binary encoded images will be removed before parsing the rtf. See `test_case_58.py`

* Now documents with multiple charsets should be parsed correctly. See `test_case_55.py`

* Support for `fonttbl` added by [sannies](https://github.com/sannies). Thanks a lot! Fixes [#53](https://github.com/joshy/striprtf/issues/53)

* Removed url hack thanks to a better regex by [stevengj](https://github.com/stevengj) also thanks to [jonas](https://github.com/joans)

* Parameter "errors" was not used anymore. Is now fixed and a test case was added. Thanks [powo](https://github.com/powo) for reporting.

* Clarified the meaning of the `encoding` parameter. It is used as the encoding if an explicit encoding is missing in the rtf file. As the majority of rtf files is encoded in ANSI, this is now the default.
Fixes [41](https://github.com/joshy/striprtf/issues/41).
Thanks to [plessbd](https://github.com/plessbd) and [stevengj](https://github.com/stevengj)