UI (details): Collapse long details

[flosell]

LambdaCD provides a way to display information on a build step in a structured way with the :details key.

However, when used extensively (e.g. long texts that might be produced by #89 or a long list of files produced by lambdacd-artifacts), it clutters up the UI in unnecessary detail.

There should be a way to collapse and expand these details.

This could be done in two ways:

• heuristic: expand/collapse automatically based on the size of the details (e.g. if the list has more than n items, if text is longer than n characters)
• user defined: Let users define the behavior with additional metadata in step results (might put too much presentation information into the step result)