Provide sane defaults for file_writer

[leblancfg]

It seems as though the line:

with libarchive.file_writer('test.tar.gz', 'ustar', 'gzip') as archive:

could be boiled down to:

with libarchive.file_writer('test.tar.gz') as archive:

as format_name and filter_name can be inferred by filepath's extension.

---

Currently, users have to:

1. Dig through ffi.py source code to see the list of valid format_names and filter_names
   • Providing a docstring for file_writer would be invaluable here.
2. Already know what types of formats to use for what filters.
   • I'm sure a list of sane default could be chosen for most formats, which could still be overwritten if specified in the function call.

I'm sure this is a jarring experience to most.

[pypingou]

Coming to this lib for the first time, I'll say +1 for this (outside of this feature more documentation would be great!)