Generic Method for Coercing Values

This uses the pattern from `Array` in #17, allowing us to provide `Coercion` behavior to any value by passing `strict = False` without writing a new C++ handler for each Value type.  This could even be made a public member, allowing savvy users to add coercion behavior to custom Value classes.  I included an example for `Bool`.

```python
from atom.api import Atom, Bool

class Test(Atom):
    b1 = Bool(0, strict=False)
    b2 = Bool(1, strict=False)

t = Test()
assert not t.b1
assert t.b2
```