In some cases, the Discord provided role_mentions and mentions array
would be empty, such as wrapping a mention with a backtick or
other frivolous characters. Since we want to completely nullify
mentions, we should not rely on the Discord provided arrays and instead
use and resolve the IDs from the content itself.
Along with this change comes with the removal of Converter.prepare and
adding two arguments to Converter.convert, the context and the argument.
I suppose an added benefit is that you don't have to do attribute
access since it's a local variable.
This allows for you to create converters that can have varying
behaviour using the converter's __init__ instead of having to do a
meta-class based approach to get around the fact that __init__ is part
of the interface.
To make up for the lack of __init__, a new method Converter.prepare was
added to do the work that __init__ used to do.
