Custom runners for non-Python code fences

[aaazzam]

This PR closes #66 by making custom runners usable for non-Python fences.

It does so by adding two opt-in paths:

1. runner:<name> now collects any fence language with the named registered runner.
2. register_runner(default_for=(...)) registers a runner as the default for specific fence languages.

Custom runners were always available a la (1) above, but only for python-based fences. Adding more opt-in paths of course adds marginal complexity to the runner selection precedence, which this PR makes:

1. Explicit runner:<name> on the fence.
2. A language match from default_for.
3. The global default runner for built-in Python fences.
4. Ignore the fence.

This PR enables us to do things like:

@register_runner(default_for=("js", "javascript"))
class JavascriptRunner(DefaultRunner):
    ...

which sets a global default for js and javascript fences

```js
expect(value).toBe(1)
```

while allowing per-fence customizeability

```js runner:OtherRunner
expect(value).toBe(1)
```

Also exposes all fence options (info-string options plus mdx-comment metadata) as an options: frozenset field on FenceTestDefinition, so custom runners can define their own per-fence flags without declaring a placeholder pytest fixture for each one.