--all-features and --no-default-features

This adjusts the code and documentation for `--all-features` and
`--no-default-features` to work correctly. With `--no-default-features`
no `DefaultAuthenticator` is made available. Users are in control of
picking the `Connector` they want to use, and are not forced to stomach
a dependency on `rustls` or `hyper-tls` if their TLS implementation of
choice doesn't happen to match one of the two.

To indicate this, the unstable `doc_cfg` feature is used to build
documentation on docs.rs. That way the generated documentation has
notices on these types that look as such:

> This is supported on crate features hyper-rustls or hyper-tls only.

Additionally this functionality is tested via additional coverage in the
Actions' CI.

.github/workflows/test.yml

@@ -4,17 +4,44 @@ name: Actions CI
 
 jobs:
   build_and_test:
-    name: yup-oauth2 
+    name: yup-oauth2
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
       - uses: actions-rs/toolchain@v1
         with:
           toolchain: stable
+          profile: minimal
+          default: true
       - uses: actions-rs/cargo@v1
         with:
           command: test
+      - uses: actions-rs/cargo@v1
+        with:
+          command: test
+          args: --all-features
+      - uses: actions-rs/cargo@v1
+        with:
+          command: test
+          args: --no-default-features --tests --examples
       - uses: actions-rs/cargo@v1
         with:
           command: build
           args: --examples
+
+  doc:
+    name: yup-oauth2
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          toolchain: nightly
+          profile: minimal
+          default: true
+      - uses: actions-rs/cargo@v1
+        with:
+          command: doc
+          args: --all-features
+        env:
+          RUSTDOCFLAGS: --cfg yup_oauth2_docsrs