Add support for <didyoumeans> section.

Hello,

This patch adds support for Wolfram|Alpha guesses at improper queries, and allows the WolframAlpha class to automatically query a didyouknow over a certain threshold (or disable that behavior).

```php
<?php
// use default threshold
new ConnorVG\WolframAlpha\WolframAlpha("8J6RWW-GX4WER9JVR");

// use custom threshold
new ConnorVG\WolframAlpha\WolframAlpha("8J6RWW-GX4WER9JVR", 0.7);

// never fallback to a didyoumean
new ConnorVG\WolframAlpha\WolframAlpha("8J6RWW-GX4WER9JVR", false);
```

It adds a new class - WADidyouknow. This is like the other WA* classes, in that they contain attributes and the tag contents.

Thanks,
Katelyn Schiesser