Basic sketch of configurable limits and sanitization

[lritter]

This PR sketches out a system where developers can configure per-element validation/sanitization to the library. This work is in response to #176

As a user of the library, a developer can add custom validation/sanitization behavior on a per "element" (overloaded term) basis. Specifically, each class defined within the library can have a "limiter" registered for it. This limiter is then used when instances of that class are serialized via as_json. These limiters can raise errors or modify the serialized data in order to help provide more robust behavior.

For example:

Slack::BlockKit::Limits.set_limiter 'Slack::BlockKit::Composition::Option', Limiters::OptionTruncateLabelAndDescriptionLimiter.new

Adds a limiter that checks select option labels, values, and descriptions for length, truncating labels and descriptions if they are too long and raising an exception if value is too long.

While this is only a sketch for now, more comprehensive default limiters can be added.