Add logout endpoint (#107)

Add logout endpoint that clears the auth cookie + optional "logout-redirect" config option, to which, when set, the user will be redirected.

internal/config_test.go

@@ -33,6 +33,7 @@ func TestConfigDefaults(t *testing.T) {
 	assert.Equal("google", c.DefaultProvider)
 	assert.Len(c.Domains, 0)
 	assert.Equal(time.Second*time.Duration(43200), c.Lifetime)
+	assert.Equal("", c.LogoutRedirect)
 	assert.Equal("/_oauth", c.Path)
 	assert.Len(c.Whitelist, 0)