generic extractor (#735)

* Generic extractor, see issue #683

* Fix failed test_names test, no subcategory needed

* Prefix directory_fmt with "generic"

* Relax regex (would break some urls)

* Flake8 compliance

* pattern: don't require a scheme

This fixes a bug when we force the generic extractor on urls without a
scheme (that are allowed by all other extractors).

* Fix using g: and r: on urls without http(s) scheme

Almost all extractors accept urls without an initial http(s) scheme.

Many extractors also allow for generic subdomains in their "pattern"
variable; some of them implement this with the regex character class
"[^.]+" (everything but a dot).

This leads to a problem when the extractor is given a url starting
with g: or r: (to force using the generic or recursive extractor)
and without the http(s) scheme: e.g. with "r:foobar.tumblr.com"
the "r:" is wrongly considered part of the subdomain.

This commit fixes the bug, replacing the too generic "[^.]+" with the
more specific "[\w-]+" (letters, digits and "-", the only characters
allowed in domain names), which is already used by some extractors.

* Relax imageurl_pattern_ext: allow relative urls

* First round of small suggested changes

* Support image urls starting with "//"

* self.baseurl: remove trailing slash

* Relax regexp (didn't catch some image urls)

* Some fixes and cleanup

* Fix domain pattern; option to enable extractor

Fixed the domain section for "pattern", to pass "test_add" and
"test_add_module" tests.
Added the "enabled" configuration option (default False) to enable the
generic extractor. Using "g(eneric):URL" forces using the extractor.

gallery_dl/extractor/2chan.py

@@ -20,7 +20,7 @@ class _2chanThreadExtractor(Extractor):
     filename_fmt = "{tim}.{extension}"
     archive_fmt = "{board}_{thread}_{tim}"
     url_fmt = "https://{server}.2chan.net/{board}/src/{filename}"
-    pattern = r"(?:https?://)?([^.]+)\.2chan\.net/([^/]+)/res/(\d+)"
+    pattern = r"(?:https?://)?([\w-]+)\.2chan\.net/([^/]+)/res/(\d+)"
     test = ("http://dec.2chan.net/70/res/4752.htm", {
         "url": "f49aa31340e9a3429226af24e19e01f5b819ca1f",
         "keyword": "44599c21b248e79692b2eb2da12699bd0ed5640a",