Changes related to #38, #39, #41 (PSL flexibility)

Make changes with the goal of more flexibility when using
`TLDExtract` (i.e. instantiating your own callable, with custom options).

Goal: control of data source & network activity, while maintaining
the nice, clean API.

* Add more tests covering most or all new cases introduced
* No regressions for existing tests
* Update docstring of `tldextract.TLDExtract`
* #38
    * Add `suffix_list_url` kwarg. Allows you to specify
the location of the Public Suffix List, or anything like
this file. This is expected to be a UTF-8 encoded text file.
    * Rename keyword argument for TLDExtract from `suffix_list_file` to
`suffix_list_url`. Update docstring accordingly. This makes it so
that it is compatible with the `fetch` keyword argument.
    * Add `fallback_to_snapshot` kwarg. Defaults to `True`.
If disabled, it will NOT use the distributed snapshot
of the PSL when `tlds` is empty.
    * README += section re: features from #38
* #39
    * Simplification of keyword arguments.
        * `suffix_list_url` for both files and public suffix list url;
        remove `Fetch`. (if you don't want it to fetch, set
        `suffix_list_url` to `False`.)
        * `cache_file` can now be set to something that evaluates to
        `False`, in order to disable caching.
    * Only complain about 1 combination of keyword arguments, where it's
necessary - and complain on construction, not calling.
    * Update README examples
* #41
    * Address concerns in Pull Request #41.
    * keep `fetch` around; if EITHER `fetch` or `suffix_list_url` are falsey,
it will disable anything that depends on `suffix_list_url` (i.e.
pulling in from the net or from local).
    *** log warning