Add support for default_host and force_host options on each client

[dbu]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Related tickets	fixes #110
Documentation	TODO (want feedback first)
License	MIT

What's in this PR?

Add new configuration options for default_host and force_host for each client.

Why?

See #110

Example Usage

    clients:
        test:
            factory: httplug.factory.guzzle6
            http_methods_client: true
            options:
                default_host: http://localhost

You can specify the httplug.client.{name}.default|force_host_plugin in the plugins list if you dont want the host plugin to come last in the chain.

Checklist

• Updated CHANGELOG.md to describe BC breaks / deprecations | new feature | bugfix
• Documentation pull request created (if not simply a bugfix)

[dbu]

just tested it in the web summercamp tutorial app and the configuration works as expected.

[dbu]

by default, we append the AddHostPlugin to the very end of the chain, but you can explicitly set it in the plugins list. i will explain this in the documentation

[Nyholm]

I do not like the new options key. I get questions like: Why isn't http_methods_client under options? How come we have both options and config?

    clients:
        test:
            factory: httplug.factory.guzzle6
            http_methods_client: true
            options:
                default_host: http://localhost
            config: 
               verify: false

Also, we should support other plugins like HeaderAppend, HeaderDefaults, HeaderRemove and HeaderSet. All these plugins (and AddHost) are very client specific. Ie, you are not likely to have the same host configured in the AddHost plugin for all clients. Compared to the CachePlugin which is very likely that you use for all clients.

Possible solution:
Maybe we should introduce two scopes of plugins. Public and Private? See this config:

httplug:
    plugins: # Public plugins
        cache:
            enabled: true
            cache_pool: 'my_cache_pool'
            stream_factory: 'httplug.stream_factory'
            config:
                default_ttl: 3600
                respect_cache_headers: true
        logger:
            enabled: true
            logger: 'logger'
            formatter: null
        redirect:
            preserve_header: true
            use_default_for_multiple: true
    clients:
        test:
            factory: httplug.factory.guzzle6
            plugins: ['httplug.plugin.logger', 'httplug.plugin.cache']
            http_methods_client: true
            config: 
                verify: false
            extra_plugins:  # Private plugins only for this client
               add_host:
                    host: http://localhost
                    replace: true
               header_append:
                   User-Agent: 'Foobar'

[dbu]

i like that idea a lot! i guess the basic question is how much we want the user to know things like adding the host are plugins, but we already went with making this transparent for the general plugins. for people not familiar with the lib, there will be a tiny confusion why something is a config and something else is an extra plugin, but i think its manageable with doc.

i guess we should also repeat the public plugins in the extra_plugins of a client. so that you can have a different redirect/log/cache/whatever behaviour. and i will keep the behaviour that you can change the order by adding the private plugins in the plugins list of the client.

[dbu]

see #112