Deprecate parameter tokens for a new trait system.

[erunion]

With the new MSON work being done in mill#42, the current parameter token system is no longer viable.

The way it was originally constructed was that it would str_replace content from the token, "{string} direction (optional) The direction that the results are sorted." onto the @api-param annotation: "@api-param:private {direction} [asc|default|desc] Sort direction". This has resulted in funky API Blueprint content being generated:

- `direction` (enum[string]) - The direction that the results are sorted. Sort direction
    + Members
        + `asc`
        + `default`
        + `desc`

With he move to MSON for parameters and representations, this format will no longer be viable because of the wide difference between MSON content and how a string replace would work.

So the solution to fix this is to construct a new "trait" system.

The basic idea is that we'd replace the parameterTokens config with traits. You would load traits with a @api-trait annotation, and then specify additional data to override that trait with, like a description, or new enum values.

• [ ] Replace the <parameterTokens> config XML declaration with a <traits> config.
• [ ] Traits will be configured as MSON within <trait> blocks:

<trait name="filter_playable">
    `true` (string, default, optional) - Choose between only videos that are playable, and only videos that are not playable.
        + Members
            - `true`
            - `false`
</trait>

• [ ] You will load traits within a resources as: @api-trait filter_playable
• [ ] Traits should support overloading data by just adding that data onto the @api-trait annotation:

@api-trait filter_playable - This is an overloaded description
    + Default: `yes`
    + Members
        - `yes`
        - `no`

• [ ] Traits should be able to be versioned.
• [ ] For response data that is only present when a trait on that resource is present, you should be able to specify this in your representation data declaration with a @api-traitEnabled trait_name annotation.
  • The use-case for this might be something like response pagination where one resource is paginated, but another isn't, despite them returning the same representation.
• [ ] In compiled API Blueprint files, traits should look as regular parameters and representation data (because essentially they are).
• [ ] Traits should support visibilities.

For some additional background, see:

• https://github.com/vimeo/mill/issues/42
• https://github.com/vimeo/mill/issues/58