Introduce alternative converter parameter in conversion method #10

Some formats have several converters, for example, docx to pdf has three of them listed below.
It should be a possibility to set up an alternative converter from the library in the Convert method.

https://v2.convertapi.com/convert/docx/to/pdf
https://v2.convertapi.com/convert/docx/to/pdf/converter/openoffice
https://v2.convertapi.com/convert/docx/to/pdf/converter/printer

This is only an example of a convert method on how it could implement alternative converters.

Default method

ConvertApi.convert('pdf', File: '/path/to/my_file.docx')

with alternative converter

ConvertApi.convert('pdf', File: '/path/to/my_file.docx', converter: 'openoffice')