Extract non-default modules to external libraries

[grych]

• Drab.Query
• Drab.Modal

Plus: write a guide on how to extend Drab.

This must be done before 1.0, as it is a significant API change.

[grych]

Rename DrabModule behaviour to Drab.Plugin

[grych]

Extract all modules, including Drab.Live and Drab.Element, to the external libraries, but add them in the dependency, and as a default plugins.

New API:

1. To include the plugin, add it to mix deps and to config as something like

config :drab, plugins: [Drab.Query, Drab.Live] ## Drab.Live and Drab.Element by default

2. Plugins from config will be default to commander, when use Drab.Commander. You may still override it with use Drab.Commander, modules: [...] for the specific Commander

Note: why? It is because most of the Drab.Plugin modules (formerly DrabModule) are injecting some JS. This is for keeping page as small as it is possible

[grych]

Moved this to the future 2.0.0, as it is not so urgent.