Provide custom ErrorCollector

#### Summary
I would like to extend the SoftProxies with custom ErrorCollector. Instead of doing actions on collected errors I want to have opportunity to perform collection based on the method name or arguments

#### Example
https://github.com/gauee/assertj-core/commit/40c4cc83d27ddb869a920ed42e039318c350541d
```java
public class ReportingErrorCollector extends ErrorCollector {
private static final Set METHODS_TO_SKIP_IN_LOGGING = ImmutableSet.of("as", "describedAs");

@Override
public Object intercept(Object obj, Method method, Object[] args, MethodProxy proxy) throws Throwable {
Object result = super.intercept(obj, method, args, proxy);
if (obj instanceof AbstractAssert && !METHODS_TO_SKIP_IN_LOGGING.contains(method.getName())) {
AbstractAssert abstractAssert = (AbstractAssert) obj;
AssertionMessage assertionMessage = new AssertionMessage();
assertionMessage.setExpected(decorateExpected(args));
assertionMessage.setActual(decorateActual(abstractAssert));
assertionMessage.setComment(method.getName());
if (!wasSuccess()) {
assertionMessage.setFailed();
}
assertionMessage.log();
}
return result;
}

private static String decorateExpected(Object[] args) {
if (CollectionUtils.sizeIsEmpty(args)) {
return StringUtils.EMPTY;
}
return CollectionUtils.size(args) == 1 ? String.valueOf(args[0]) : Arrays.toString(args);
}

private static String decorateActual(AbstractAssert abstractAssert) {
return abstractAssert.actual == null ? StringUtils.EMPTY : abstractAssert.actual.toString();
}
}

```

#### Java 8 specific ?
* YES : create Pull Request from the `master` branch

Comienza localizando SoftProxies y ErrorCollector; después inspecciona el método intercept y compara el comportamiento con el mostrado en el commit de ejemplo referenciado. La tarea estará terminada cuando los llamadores puedan proporcionar un collector personalizado que pueda basar las decisiones de recopilación en el nombre o los argumentos del método de aserción.

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