Change BlockProcessor to Blockprocessor

[venthur]

trafficstars

this makes it consistent with the spelling of the other processors.

This PR also adds an alias of the old BlockProcessor to the new Blockprocessor for backwards compatibility.

[facelessuser]

I imagine the old name should be marked as deprecated via PEP 562.

[waylan]

While I understand the motivation for making this change I do have some apprehension about accepting it. Specifically, by proposing this you have ignored the suggestion in our contributing guidelines:

Legacy code which does not follow the guidelines should only be updated if and when other changes (bug fix, feature addition, etc.) are being made to that section of code. While new features should be given names that follow modern Python naming conventions, existing names should be preserved to avoid backward incompatible changes.

In other words, I would be more emendable to this change if we were making a change to the class itself. Of course, there would need to be a real need for the change, not just an arbitrary change.

Regardless, before this change could be accepted, a note would need to be made in the release notes and the old class would need to be marked as deprecated (note that a deprecated decorator exists at markdown.util.deprecated). Also, a change like this would need to be part of a new point release (rather than a bug-fix release), which I don't expect to happen for a while (we just released 3.4.0). Therefore, even if this PR was accepted, it wouldn't likely be merged until we were ready to start development in earnest on 3.5.0.