Clear via type's Clear method

[bddckr]

The current implementation of the ClearPropertyMethod weaver only sets a reference type property to null.

The task is to instead do the following (in order):

1. Call the getter to retrieve the current value.
2. Compare the value against null/the type's default value.
3. If it's null/default return early.
4. If it's not null/default check if the type has a public void Clear() method.
5. Call that Clear method on the value if found.
6. If not found continue doing the existing property = default; instead.

Also update the documentation found in the Readme.

[bddckr]

Closing in favor of #25.