Handle checks and results as a structured array instead of an array of strings.

What?

Currently, the errors produced by the check class are a list of strings containing HTML tags. This makes using these messages difficult for something that's not printing the results to an HTML page.

Exampe:

"<span class="tc-lead tc-warning">WARNING</span>: <strong>screenshot.png</strong> is 704.2 KB in size. Large file sizes have a negative impact on website performance and loading time. Compress images before using them."

If we the plugin can handle the messages as a list of objects (PHP relational arrays) and format just before outputting them, it would be easier to handle that in different formats (Html page, console output, JSON).

Example of 2 different outputs being implemented (console and json in the console): https://github.com/WordPress/theme-check/pull/458

Expected

The plugin could handle the errors like this. In this way, each output handler could format it accordingly without parsing or removing HTML.

$checks = array (
    array(
         'type' => 'ERROR',
         'message' => 'This is an error text'
    ),
    array(
         'type' => 'WARNING',
         'message' => 'This is an warning error text'
    ),
    array(
         'type' => 'INFO',
         'message' => 'This is an info text'
    ),
)

No files or tests are named, so start by tracing the plugin's current check and result flow. Review the console and JSON examples in PR #458, then verify that checks retain structured type/message data and that each output format can render it without parsing HTML.