Setting a custom OkHttpClient

**Is your feature request related to a problem? Please describe.**
I would like to set a custom OkHttpClient when calling ` pdfView.initWithUrl()` to be able to add other properties to the client. For me, just setting the headers is not enough.

**Describe the solution you'd like**
The `pdfView.initWithUrl()` should have a parameter called httpclient that would be passed in for the PdfDownloader
