Hit test - status update

Hey all,

Quick status update on hit-test repository
<https://github.com/immersive-web/hit-test>:
I have updated the explainer
<https://github.com/immersive-web/hit-test/blob/master/hit-testing-explainer.md>
to address the concerns with subscribing to transient input sources -
proposed solution allows the application to specify to which transient
input sources it wants to subscribe by passing input source's profile
- see this
section
<https://github.com/immersive-web/hit-test/blob/master/hit-testing-explainer.md#pre-registration-for-transient-input-sources>
for more details. I have also added an option for the application to
pre-filter what kind of entities are supposed to be used when computing hit
test results - see this section
<https://github.com/immersive-web/hit-test/blob/master/hit-testing-explainer.md#limiting-results-to-specific-entities>
for more details.

Please take a look and file issues in the repo to provide feedback on the
current proposal! I'm going to start drafting a spec for hit-test in the
near future (starting with the more stable parts unrelated to transient
input sources), so the sooner I hear your feedback, the better!

Thanks,
-Piotr