[plugin] Move the plugin loading from plugin_handler to plugin.

Before:

+ The plugins were loaded one after the other in plugin_handler

Now:

+ The plugins are all loaded at once in config/plugin

Cons:

+ The more plugins will be added, the more time it will takes at the beginning
to load all of them (for the current number of plugins, the user does not feel
any lag).

Pros:

+ It becomes easier to declare new attributes for each plugin (like
DESCRIPTION, USE_INTERNET_RESOURCES, a new class, etc.)

TODO:

+ Take into account the OWTF command line's options like -t, -e and -o in order
not to load all the plugins, but only the required ones.