Refactor gofaxsend into a package

[EtienneBruines]

Using the idiomatic Go convention of writing code in a package,
and then handle the CLI in a smaller main-package.

This makes the code importable from other parts of the code and
more maintainable.

In the spirit of portability, also split SendQfile into a separate
function that uses the Qfile reference without requiring the use
of the filesystem.

[denzs]

Thanks for your commit!
If you like, add another PR and add yourself to the AUTHORS file :)