Expose enableXfa param

[bitforcesrl]

- [ ] Regression (a behavior that used to work and stopped working in a new release)
- [ ] Bug report -> please search issues before submitting
- [X] Feature request
- [ ] Documentation issue or request

Currently, the library loads the document using pdfjs with the parameter enableXfa: true. It would be useful to expose this parameter as a component input to allow the possibility of disabling this feature.

ng2-pdf-viewer/src/app/pdf-viewer/pdf-viewer.component.ts

Line 493 in e15fea1

enableXfa: true,